There was an issue from about 4.14 to 4.30 with the mp10 where if the smtp server was outside of the local lan then the email would not go through.
There was a 4.30l release here to fix this and it was also fixed in 4.4x.

What does the port activity screen show during this?

Depending on the size of the message it can take a while for it to convert the g.729 (mp10) or g.726 (mp10a, os7030, mp20s) file to a wav file and send the email