Cannot send emails; Error message: 550 relaying mail not allowed

Error message says: System Administrator
Undeliverable:
Your message did not reach some or all of the intended recipients.
The following recipient could not be reached:
550 relaying mail to aol.com (or yahoo.com or whomever I'm sending to) is
not allowed.

Thanks for the reply Hal Hostetler, and I was trying to reply to your reply,
but for some reason, it wasn't working.

Also, is there a live telephone number for this problem that I can talk with
someone to walk me through this problem in case it's too complicated? I'm
really not technically savvy, very intimidated by the computer when it
doesn't work and I don't know how to make it work, although I need it fixed
very urgently and haven't been able to find the Technical Support number on
my own? Sorry!!!

Sincerely,

Marvel