The sender address was invalid

You haven't said what email provider you are using. That's critical information.

I'm guessing that you are using an email account from your home Internet provider, and are attempting to connect to their SMTP server to send mail.

Many ISPs check email senders to make sure that they are accessing the server from within their own network. 3G = not from within your home Internet provider's network!

There are a few solutions:

1. See if your home Internet provider permits sending mail from outside their own network. If they do, you probably will only be permitted to do so using an authenticated connection (home Internet providers often permit unauthenticated access to SMTP, with the provision that you need to connect from within their network) and probably will require an encrypted connection on a different port. (Which you need to use for your own protection in any case).

2. Use your cell phone provider's SMTP server. However, you could have the same problem issue when sending mail from WiFi...

3. Use a third-party mail provider. Most offer encrypted SMTP connections with authentication that you can access from anywhere. (I use Rackspace Mail. I've had the service forever, and it's RackSpace Mail because a big fish bought a little fish that bought a smaller fish.... and I'm grandfathered at $1/month.)