Can't send attachments using Network Solutions domain-email??

My phone line is clear, and my ISP (Earthlink) even supplied me with new DNS numbers, but it didn't help. I cannot send email with an attachment through my Network Solutions (domain) email.

I tried two different mail agents (Apple mail, Thunderbird ), using my personal domains smtp for the outgoing server, and Thunderbird responded with "your server at xxxx@ mydomain .com rejected your mail server at smtp. mydomain .com". Apple Mail rejects also, error msg states: "CANNOT USE THE SERVER XXXX@mydomain.com~
Sending the message content to the server failed.

Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be delivered.~"



So I'm really confused now, since I see that several of the posts here are saying about the same thing, but with different servers, like comcast, etc.

Now I'm wondering: Is it Leopard 10.5.5 that is creating this problem with the mail even though I am trying different mail programs? Apple voodoo?

BTW, I can send and receive small emails sans attachments to and from all of the programs, including apple mail, but its when I attach ANYTHING of size from 100kb to 1.5 mg that apple mail gives the error message ~

Also, FYI, the mail activity monitor progress meter starts up like its sending, then drops off to 0KB/s and just spins.

(Please excuse the similarity to another post, but I thought it might be under the wrong thread.)