There could be a few possibilities:
1. How is your SMTP server configured in Aquamail? If it is configured with no authentication, that would not work from any network but provider's own network. In that case, choose to use authentication.
That may require using a different port (587 or 465, depending on your provider's configuration, - instead of port 25).
2. Some mail providers block connections from mobile networks. This is especially the case if you are connecting to port 25 (as opposed to 587 or 465).
A major Italian ISP was reported here recently to do that.
To check your SMTP server configuration, - long-touch on the account in the account list view, and then "account setup", choose "manual".
3. Who is your mail provider? A few users have reported some problems similar to yours with Yahoo Mail.
If none of these suggestions are applicable/help, please post your SMTP server settings in Aquamail (without username/password).
