Comment 5 for bug 139071

552 is standard <http://www.greenend.org.uk/rjk/2000/05/21/smtp-replies.html> but there are many broken servers out there, so you can't reply on it.

We cannot rely on the first server rejecting it.

I think we probably need to cap it at some reasonable size (1MB?) if only out of consideration for the final user; many clients will be slow with very large messages.