here is a transcript of 2 SMTP sessions. One was made using Domino 5.0.12 SMTP server, another using Domino 6.5.1 (user and server names changed)
220 myserver.mydomain.com ESMTP Service (Lotus Domino Release 5.0.12) ready at Wed, 11 Feb 2004 12:58:58 +0100
helo me
250 myserver.mydomain.com Hello me ([XXX.XXX.XXX.XXX]), pleased to meet you
250 nobody@hotmail.com… Sender OK
rcpt to:my@email.address
250 my@email.address… Recipient OK
data
354 Please start mail input.
test
.
250 Mail queued for delivery.
220 anotherserver.mydomain.com ESMTP Service (Lotus Domino Release 6.5.1) ready at Wed, 11 Feb 2004 15:02:09 +0300
helo me
250 anotherserver.mydomain.com Hello me ([XXX.XXX.XXX.XXX]), pleased to meet you
250 nobody@hotmail.com… Sender OK
rcpt to:my@email.address
250 my@email.address… Recipient OK
data
354 Please start mail input.
test
.
250 Mail queued for delivery.
As you can see the session’s data and headers are the same, and both looks like an example from RFC821
Both e-mails was accepted and delivered. Only difference that body item was not created for message which was sent via Domino 6.5.1 and even View-Show-Page Source menu command is not available for this message.
Other users already reported problems with Domino 6.5 SMTP :
P.S. Probably it’s time to create STC on the Passport site 
MIME