SMTP 554 5.7.1 Error

Hi. Haven't been able to find too much info in terms of a layman's fix and my ISP is not much help. I have a business acquaintance that has been trying to email us business info from his work email as well as his AOL account. He keeps getting a bounce back with the message referring to SMTP 5545.7.1 (P4).

From what I can tell it has something to do with authentication when I google that error code. But don't know if it's on my end or his. Everything else works fine for both of us.

He doesn't get our emails and we can't receive his. I am using Thunderbird and my service is through Centurylink.

Thanks,
Jay

Is it attachments that are failing? Can the business acquaintance send a regular email to you with no attachments?

Unfortunately it's everything. I had him send an email with subject line test only and it failed. He sent a test to my work email and that got through. So thinking it may be Centurylink after all which is our home email provider.

Every mail server requires its users to provide a username and password to send mails. This is to keep spammers out. But very often valid mail users forget to turn on authentication in their mail clients, and the MX rejects their mail.
A quick look at the support tickets shows that 95% of ‘Relay Access Denied’ errors are caused by incorrect SMTP settings.
So, when customers come to us facing this error, the first thing we check is their mail client settings. By guiding users with the correct settings specific to each mail client and mail server, we help them send emails without this error.