smtp: Fixed sending of double CRLF caused by first in EOB

If the mail sent during the transfer contains a terminating <CRLF> then
we should not send the first <CRLF> of the EOB as specified in RFC-5321.

Additionally don't send the <CRLF> if there is "no mail data" as the
DATA command already includes it.
smtp: Updated RFC-2821 references to RFC-5321
RELEASE-NOTES: synced with 92ef5f19c801