Sonic.net

Ok, I don't do this kind of stuff much but I want to be able to send from the email address I actually use instead of the @sonic.net, and I could log in using that email but when I tried to send a message to see if it was working it told me "autheication failed " (I think) and wouldn't send the message. BLAAAA

Nope, you need to log in using the mailbox name, not the email address.

To send from an alternative email address, use Settings > Identities, and add a new identity. You can set the display name, default BCCs, and whether that email address should be the default for "from."