iPhone 3G Cannot Send Mail. sender address was invalid.

This error message is not accurate - the sender address is invalid - with you being the sender.
This error message means there is a problem with the accounts SMTP server. Do you know if the accounts SMTP server is authenticated? If you were able to send messages with this SMTP server regardless the internet service provider being used for your internet connection at the time, more than likely the SMTP server is authenticated.
If you are able to delete the SMTP server only and recreate it, try doing so. If not, try deleting and recreating the email account on your iPhone.
 MacBook Pro 15" Intel Core 2 Duo/2.53GHz/4GB RAM/500GB HD Mac OS X (10.6.2)  iPod w/Video 30GB,  iPhone 3G 16GB

The configuration and use of any outgoing server address (SMTP) may not be supported due to several factors including, but not limited to, the inability for the outgoing server (SMTP) to authenticate users (whether by IP or username/password) that are not directly connected to that Internet Service Providers(ISPs) network.
This is usually your HOME wi-fi and you are not connected to it because you are on your iPhone away from the house. This is mainly done to prevent unsolicited users from sending SPAM via the ISPs servers.
An alternative is to use ATT SMTP Server (for US) when you are on the 3G or data network for the carrier and not on your home wi-fi. So enable ATT SMTP server under Other SMTP servers under your Mails account settings on the iPhone.
If you are interested you can obtain that information directly from the email provider. This article has a great temple that you can use to ask all necessary information from the provider including If you use your ISP’s own email service (instead of a third-party such as Gmail), sometimes it may not work if you use another Internet access point, such as a cafe with wireless Internet. Ask your email provider if the settings they give you will work on other networks.
What to ask if youre having issues sending, receiving, or configuring Mail
http://support.apple.com/kb/HT1277
 iMac Mac OS X (10.6.2)  iPhone 3GS