The size is probably the issue. An attachment adds to the size about 35%
due to encoding, so your 5.10MB file is now roughly 7MB. You can try the
following, but it will only help if your server allows a message of that
size.
In Tools | Accounts | Mail | Properties | Advanced, set the time-out
slider to the max.
Turn off e-mail scanning in your anti-virus program. It is a redundant
layer of protection that eats up CPUs, slows down sending and receiving,
and causes a multitude of problems such as time-outs, account setting
changes and has even been responsible for loss of messages. Your
up-to-date A/V program will continue to protect you sufficiently. For
more, see:
http://www.oehelp.com/OETips.aspx#3
Why you don't need your anti-virus to scan your email
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm
Note that for some AV programs, it may be necessary to uninstall the
program and reinstall in Custom Mode and uncheck e-mail scanning when the
option arises.