I keep getting a error message stating "550 (the email address here &) relaying not allowed.
This has been going on for a month now. Sometimes they go though, sometimes they don't. What do you think is the best way to decipher what the problem may be. I tried sending through thunderbird, but that came back as as undeliverable also.
Any thoughts?

Error 550 means that you are connecting via one ISP, but using an email server (SMTP server) belonging to another ISP.

This commonly happens afetr you change ISP's (Internet Service Providers) and forget to change the SMTP server names.

Some ISP's may still "tolerate" this but most return error 550, as it is a protection from spammers using another innocent email server to deliver their rubbish, as an example.

Check that your SMTP and POP3 settings match those required (and usually published) by your ISP.