"can't sent large e-mails"
Do you really want answers as vague as your question? So how do YOU
know that "large" e-mails are not getting sent? Then tell us how you
know, like the item sticks in the Outbox, an error message pops up or
is shown in the progress dialog, or however you have determined that
e-mails are not getting sent. We aren't there looking over your
shoulder to see what you didn't tell us.