I have read similar problems and all sorts of ways around but my system is awful. Even small e-mails can get stuck in the outbox. I can send nothing bigger than 1MB, my broadband is hopeless - I have 1mb speed so do I assume that thats the largest I can send or is there a simple way around this?
I have searched on 'programmes' for microsoft but cant find it. My system is run on BT.
Have you tried sending email via your webmail? E.g. I'm with BT which means I can access my email via Yahoo, so you could try sending a larger email via webmail and see if it works? In which case that would suggest Outlook (or whatever email programme you're using) is the issue.
If it is Outlook, try running it in safe mode. I think you do that from the Start menu, putting outlook.exe in the Run prompt. But maybe someone else here can advise (or google it)