Exhibit 10.21

FIFTH AMENDMENT TO LEASE AGREEMENT

THIS FIFTH AMENDMENT TO LEASE AGREEMENT (the "Amendment") made this
20th day of August, 2006 between DAN ROAD BUILDING LLC, a Delaware limited liability company, with an address in care of Great Point Investors LLC, Two Center Plaza,
Suite 410, Boston, MA 02108 ("Landlord") and NETWORK ENGINES, INC., a Delaware corporation, with an address of 15 Dan Road, Canton, MA
02021 ("Tenant").

WHEREAS,
New Boston Batterymarch Limited Partnership, Landlord's predecessor-in-interest, and Tenant entered into a Lease Agreement dated October 19, 1999,
as amended by a First Amendment to Lease Agreement dated February 1, 2000, a Second Amendment to Lease Agreement dated June 1, 2000, a Third Amendment to Lease Agreement dated
September 14, 2000 and a Fourth Amendment to Lease Agreement dated October 20, 2003 (as amended, the "Lease"), pursuant to which Tenant
leases approximately 51,935 rentable square feet at 15 Dan Road, Canton, Massachusetts (the "Premises");

WHEREAS,
Tenant desires to extend the initial term of the Lease for the Premises through March 31, 2009, and Landlord has agreed to such extension on the terms and conditions set
forth herein; and

WHEREAS,
Landlord and Tenant desire to memorialize their understanding and modify the Lease consistent therewith.

NOW,
THEREFORE, in consideration of the mutual covenants herein contained, and other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged,
Landlord and Tenant agree that the Lease shall be amended as follows:

1.Term.    The Lease Term shall be extended so as to include the period from March 31, 2007 through
March 31, 2009.

2.Rent.    Annual base Rent for such extended period shall be as follows: April 1, 2007 through
March 31, 2008: $662,171.25 per annum ($55,180.94 per month); and April 1, 2008 through March 31, 2009: $714,106.25 per annum ($59,508.85 per month). Notwithstanding the
foregoing, Tenant shall not be required to pay base Rent for the month of April, 2007.

3.Security Deposit.    As a condition to the effectiveness of this Amendment, on or before October 31,
2006, Tenant shall deliver to Landlord an amendment to Letter of Credit No. SVB00IS1965 issued by Silicon Valley Bank, modifying the expiration date of such Letter of Credit to April 30, 2009.

4.Renewal Option.    Section XLX of the Lease is hereby replaced with the following with respect to Tenant's
option to extend the Term of the Lease beyond March 31, 2009:

"Provided
that Tenant is not in default beyond any applicable grace or cure period hereunder, Tenant shall have the right to extend the term of this Lease for one (1) additional
term of two (2) years (the "Extended Term") commencing on April 1, 2009. Tenant shall exercise the right to extend the term of this Lease
by written notice to Landlord no later than June 30, 2008. The Rent for the Extended Term shall be the greater of the then current Rent or the Market Rent as hereinafter defined. The
"Market Rent" shall be determined by Landlord based on leases then currently under negotiation or recently executed for the Property. If no such leases
are under negotiation or have been recently executed, then the Market Rent shall be reasonably determined by Landlord based on comparable office space in the town or city in which the Premises are
located (if any such space exists) taking into account, among other factors, amenities, setting, location and demographics. Landlord shall notify Tenant of the Market Rent within a reasonable period
after Tenant notifies Landlord that Tenant is exercising the option to extend, provided Landlord shall not be required to set the Market Rent prior to ten (10) months before the expiration of
the Term. If Tenant disagrees with Landlord's

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determination
of the Market Rent and the parties are unable to agree upon a Market Rent within thirty (30) days after Landlord's notice thereof, then the Market Rent shall be determined by
appraisal made as hereinafter provided by a board of three (3) reputable independent commercial real estate brokers, each of whom shall have at least ten (10) years of experience in the
Greater Boston office rental market and each of whom is hereinafter referred to as an "appraiser." Tenant and Landlord shall each appoint one such appraiser and the two (2) appraisers so
appointed shall appoint the third appraiser. The cost and expenses of each appraiser appointed separately by Tenant and Landlord shall be borne by the party who appointed the appraiser. The cost and
expenses of the third appraiser shall be shared equally by Landlord and Tenant. Landlord and Tenant shall appoint their respective appraisers at least five (5) months prior to commencement of
the period for which Market Rent is to be determined and shall designate the appraiser so appointed by notice to the other party. The two appraisers so appointed and designated shall appoint the third
appraiser at least four (4) months prior to the commencement of such period and shall designate such appraiser by notice to Landlord and Tenant. The board of three (3) appraisers shall
determine the Market Rent of the space in question as of the commencement of the period to which the Market Rent shall apply and shall notify Landlord and Tenant of their determinations at least sixty
(60) days prior to the commencement of such period. If the determinations of the Market Rent of any two (2) or all three (3) appraisers shall be identical in amount, said amount
shall be deemed the Market Rent of the subject space. If the determinations of all three (3) appraisers shall be different in amount, the average of the two (2) values nearest in amount
shall be deemed the Market Rent. The Market Rent of the subject space determined in accordance with the provisions of this Section shall be binding and conclusive on Tenant and Landlord. All of the
covenants, agreements, terms and conditions contained in this Lease shall apply to the Extended Term."

5.Condition of the Premises.    Tenant agrees that it has accepted, and will continue to accept the Premises in
its "as is, where is, with all faults" condition, and without any express or implied representations or warranties of any kind (including, without limitation, any warranties of merchantability,
fitness or habitability). No agreement of Landlord to alter, remodel, decorate, clean or improve the Premises (or to provide Tenant with any credit or allowance for the same) has been made by or on
behalf of Landlord or relied upon by Tenant, and no representation regarding the condition of the Premises or the suitability of the Premises for Tenant's proposed use thereof have been made by or on
behalf of Landlord or relied upon by Tenant.

6.Brokers.    Each party represents and warrants that except for Cushman & Wakefield and CB Richard
Ellis/New England (the "Brokers"), it has dealt with no broker, agent, or other person in connection with this transaction and that, except for the
Brokers, no broker, agent or other person brought about this transaction and each party agrees to indemnify and hold the other harmless from and against any claims by any other broker, agent or other
person claiming a commission or other form of compensation by virtue of having dealt with the indemnifying party with regard to this leasing transaction. The provisions of this paragraph shall survive
the termination of the Lease.

7.Incorporation.    In all other respects, the terms and provisions of the Lease are ratified and reaffirmed
hereby, are incorporated herein by this reference and shall be binding upon the parties to this Amendment. Tenant acknowledges that as of the date of this Amendment, Tenant (a) is not in
default under the terms of the Lease; (b) is not aware of any defense, set-off or counterclaim to the enforcement by Landlord of the terms of the Lease; and (c) is not aware
of any action or inaction by Landlord that would constitute a default under the Lease.

8.Definitions.    All capitalized terms used and not otherwise defined herein, shall have the meanings ascribed to
them in the Lease.

SIGNATURES
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IN
WITNESS WHEREOF, Landlord and Tenant have caused this Amendment to be duly executed, under seal, by persons hereunto duly authorized, in multiple copies, each to be considered an original hereof,
as of the day and year first above written.

|                      |  |                       |  |                   |  | 
|--------------------- |  | --------------------- |  | ----------------- |  | 
|LANDLORD:             |  |                      
|DAN ROAD BUILDING LLC |  |                      
|By:                   |  |                       |  |                  
|                      |  | Name:                 |  |                   |  | 
|                      |  | Title:                |  |                   |  | 
|TENANT:               |  |                      
|NETWORK ENGINES, INC. |  |                      
|By:                   |  | /s/ Douglas G. Bryant |  |                  
|                      |  | Name:                 |  | Douglas G. Bryant |  | 
|                      |  | Title:                |  | CFO               |  | 

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