<PAGE>

                                                                  Exhibit 10.21

                                    SUBLEASE

         THIS AGREEMENT made as of the 1st day of April, 2001, by and between
SBC SERVICES, INC., a Delaware corporation ("Sublessor") and PRODIGY
COMMUNICATIONS L.P., a Delware limited partnership ("Sublessee");

                                   WITNESSETH:

         WHEREAS, Sublessor leases 112,708.5 rentable square feet (shown on
the attached EXHIBIT A) of space (the Premises"), comprising the entire
building known as River Place Pointe III, 6500 River Place Boulevard, Austin,
Texas (the "Building") pursuant to a certain lease dated as of July __, 2000
(the "Lease"), made by and between Investors Life Insurance Company of North
America, as landlord ("Landlord") and Sublessor, as tenant, which Lease is
attached hereto as EXHIBIT B and made a part hereof; and

         WHEREAS, Sublessee wishes to sublet the entire Premises (sometimes
also referred to as the "Sublet Premises") from Sublessor on an "absolutely
net" basis;

         WHEREAS, Sublessor wishes to sublet the Sublet Premises to Sublessee
on an "absolutely net" basis upon certain terms and conditions;

         NOW, THEREFORE, the parties hereto, for themselves and their
respective successors and assigns, in consideration of the Sublet Premises
and the covenants hereinafter contained and the sum of TEN DOLLARS ($10.00)
to each party paid by the other, the receipt of which is hereby acknowledged,
do covenant and agree as follows:

1. Sublessor does hereby sublet to Sublessee the Sublet Premises on the terms
and conditions set forth in the Lease and this Sublease on an "absolutely
net" basis for the term commencing as of April 1, 2000 and ending on the day
immediately prior to the "Expiration Date" of the Lease, as defined in the
Lease (the "Sublease Expiration Date") unless sooner terminated under the
terms of the Lease or this Sublease. THIS SUBLEASE IS ENTERED INTO ON AN
"ABSOLUTELY NET" BASIS SUCH THAT ANY AND ALL COSTS AND EXPENSES INCURRED BY
SUBLESSOR IN CONNECTION WITH THE LEASE OR THIS SUBLEASE SHALL BE PAID BY
SUBLESSEE UPON DEMAND MADE BY SUBLESSOR.

2. Sublessee covenants and agrees to pay directly to Landlord during the term
of this Sublease, commencing on the "Commencement Date" of the Lease, as
defined in the Lease: (a) the Annual Base Rent which Sublessor is required to
pay to Landlord under the Lease, payable in equal monthly installments not
later than ten (10) days prior to each date when Sublessor shall be required
under the Lease to pay the same to Landlord; (b) the Annual Additional Rent
which Sublessor is required to pay to Landlord under the Lease, payable in
equal monthly installments not later than ten (10) days prior to each date
when Sublessor shall be required under the Lease to pay the same to Landlord;
(c) as additional rent, such other sums of money as shall become due

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                                                                  Exhibit 10.21

and payable by Sublessor to the Landlord pursuant to the Lease; and (d) all
premiums and other sums which Sublessor pays in order to obtain, maintain,
substitute or renew the "Bond," as defined in Section 28 of the Lease.
Furthermore, Sublessee shall provide Sublessor with written evidence of its
payment of amounts required to be paid pursuant to Paragraph 2(a) and (b)
hereof simultaneously with payment of such amounts to Landlord.

3. If Sublessee fails to pay the amounts set forth in Section 2(a) and (b) by
the dates required by Section 2(a) and (b), then (a) Sublessee shall be in
default under this Sublease; (b) Sublessor shall be entitled to pay the same
and Sublessee shall reimburse Sublessor on demand for such payment; and (c)
Sublessee shall pay Sublessor ten percent (10%) of the overdue payment.

4. The amounts set forth in Paragraph 2(c) and (d) hereof shall be paid to
Sublessor at the address set forth in Paragraph 11 of this Sublease, in
advance, on the first day of each and every month throughout the term of this
Sublease. If Sublessee fails to pay any such amounts within five (5) days
after same is due, such unpaid amount shall bear a ten percent (10%) late
charge, which will be immediately due and payable. For any partial month,
rent shall be prorated.

5. Sublessee shall not have the right to (a) assign or transfer any interest
in this Sublease; (b) further sublet the Sublet Premises; nor (c) use the
Sublet Premises for any purpose other than as permitted in the Lease. If
Sublessee is a corporation, the transfer of in excess of twenty-five percent
(25%) of the capital stock or voting rights of Sublessee shall be deemed an
assignment of this Sublease.

6. Sublessee shall maintain the insurance required in Paragraph 12(a) of the
Lease, provided, however, that all rights and benefits specified for Landlord
pursuant to Paragraph 12 shall apply to and inure to the benefit of Landlord
and Sublessor, provided, however, that the insurance coverage which Sublessee
is required to obtain and maintain pursuant to Paragragh 12(a)(i) of the
Lease shall include Sublessor as a NAMED INSURED. Upon the execution of this
Sublease, Sublessee shall deliver to Sublessor a certificate of insurance
evidencing the required insurance coverage.

7. Sublessor shall in no event be obligated to perform Landlord's obligations
under the Lease. Sublessor shall cooperate with Sublessee and use
commercially reasonable efforts to enforce the terms, covenants, obligations
and conditions on the part of, or to be performed by, Landlord under the
Lease, provided however that any reasonable costs actually incurred by
Sublessor for such cooperation or efforts shall be paid by the Sublessee.

8. Except for the initial tenant improvement work to be performed by the
Landlord, Sublessee agrees to accept the Sublet Premises in "AS IS"
condition. Sublessee acknowledges that no other representations, express or
implied, with respect to the condition have been made to the Sublessee by the
Sublessor.

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                                                                  Exhibit 10.21

9. In the event that Sublessee (a) fails to fulfill any of its obligations
under this Sublease; (b) fails to perform any of the Lease obligations; or
(c) shall be adjudged bankrupt or insolvent or shall make an assignment for
the benefit of creditors, or if a receiver or trustee of the Sublessee's
property shall be appointed and not discharged within six (6) months, such
occurrence shall be an event of default and Sublessor shall have any and all
rights and remedies available to it at law and equity, including the rights
and remedies of Landlord described in the Lease.

10. Sublessee shall indemnify Sublessor and save Sublessor harmless from and
against any and all claims, liability, and expense for loss or damage,
including indirect, special or consequential damages, suffered by Sublessor
because of (i) the negligence or willful misconduct of Sublessee, its agents,
contractors, employees, visitors or invitees; (ii) any act or occurrence in
the Sublet Premises, unless caused solely by the negligence or willful
misconduct of Sublessor, its agents, contractors, employees, visitors or
invitees; (iii) breach of this Sublease by Sublessee, its agents,
contractors, employees, visitors or invitees; or (iv) the holding over by
Sublessee after the expiration of the term of this Sublease. Sublessee's
obligations under this Paragraph 10 shall survive the termination of the
Sublease.

11. Any notice, approval, request, consent, bill, statement or other
communication required or permitted to be given, rendered or made by either
party hereto, shall be in writing and shall be sent to the parties hereto by
certified United States mail, postage prepaid, return receipt requested, or
by nationally recognized overnight courier providing evidence of delivery, at
the following addresses:

          Sublessor:    SBC Services, Inc.
                        1010 N. St. Mary's Street - Room 5-U-04
                        San Antonio, Texas 78215
                        Attn: J. Stephen Sundby, Director of
                        Transactions, South West

          With a copy
          to:           Kenneth H. Gitter, Esq.
                        General Attorney - Real Estate
                        SBC Communications, Inc.
                        175 E. Houston
                        San Antonio, Texas 78205

          Sublessee:    Prodigy Communications L.P.
                        6500 River Place Blvd., Bldg. III
                        Austin, TX  78730
                        Attn: Vice President, Human
                        Resources and Administration

          With a copy
          To:           Dan Iannotti, Esq.
                        General Counsel

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<PAGE>

                                                                  Exhibit 10.21

                        Prodigy Communications L.P.
                        6500 River Place Blvd., Bldg. III
                        Austin, TX  78730

12. This Sublease shall, at Landlord's option, be subordinate or superior to
the lien of any mortgage covering the Building. Sublessee shall execute
promptly any instrument that Landlord may request to confirm that this
sublease is superior or subordinate in lien to any mortgage pursuant to the
foregoing provisions of this paragraph. In the event of a foreclosure of a
mortgage covering the Building, which mortgage is pursuant to the foregoing
provisions of this paragraph, prior in lien to this sublease, then
notwithstanding such foreclosure, Sublessee shall not disaffirm this sublease
or any of its obligations hereunder and at the request of the mortgagee or
the purchaser at the foreclosure sale, Sublessee shall attorn to such
mortgagee or purchaser and if required, execute a new sublease for the
Premises on all the terms and conditions as herein contained, except that the
term of such new sublease shall be for the balance of the term of this
sublease.

13. Sublessee acknowledges that where consent of Landlord is required under
the Lease, Sublessor's consent is also required.

14. The Sublessor, its agents, employees, prospective purchasers, prospective
mortgagees and prospective lessees may, from time to time, enter the Sublet
Premises during the term of this Sublease.

15. If any term or provision of this sublease or the application thereof to
any person or circumstance shall to any extent be invalid or unenforceable,
the remainder of this sublease or the application of such term or provision
to persons or circumstances other than those as to which it is invalid or
unenforceable, shall not be affected thereby; and each term and provision of
this sublease shall be valid and be enforceable to the fullest extent
permitted by law.

16. Each party represents to the other that no broker participated in the
negotiations leading to the Sublessee's rental of the Sublet Premises from
the Sublessor. Each party agrees to indemnify and hold the other party
harmless from and against any claim or demand of any broker or agent who
claims that he/she participated with that party in this transaction.

17. Excluding the matters contained herein which will be governed by this
Sublease, Sublessee agrees (i) to be bound to both Sublessor and Landlord by
the terms and conditions of the Lease, and (ii) to perform all of Sublessor's
obligations under the Lease.

         IN WITNESS WHEREOF, the parties have hereto executed this Sublease
on the date above written.

WITNESSES:                                 SUBLESSOR:
                                           SBC SERVICES, INC.

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                                                                  Exhibit 10.21

----------------------------
                                           By:
                                                    ----------------------------
                                                    Name:
----------------------------                        Title:
                                                    Date:

                                           SUBLESSEE:
                                           PRODIGY COMMUNICATIONS L.P.

                                           By:      PRODIGY COMMUNICATIONS
                                                    CORPORATION, GENERAL
                                                     PARTNER





                                          By:
                                             ----------------------------------
                                             Name:
                                             Title:
                                             Date:




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<PAGE>

                                                                  Exhibit 10.21

                                    EXHIBIT A

                                   FLOOR PLAN


<PAGE>

                                                                  Exhibit 10.21

                                    EXHIBIT B

                                      LEASE