Sendmail Error Solved: "550 Relaying denied."
After having so-called Sendmail experts install and configure Sendmail v8.14.3 along with Dovecott, I found that email sent outside the server from a mail client (such as Outlook) would receive one of the followng "550 Relaying denied" errors "Relaying denied. Proper authentication required.", or "550 Relaying denied. IP name lookup failed", or "550 Relaying denied. IP name possibly forged.".
My experts gave me the run-around, and claimed it was because of my ISP, or a spam filter, etc. The truth is, this sendmail error message is because they're not really experts.
Beginning with Sendmail v8.9, open mail relay is disabled by default and you will receive a "550 Relaying denied" error when you send mail to anyone outside of your host domain. To correct this problem, you can add domains to your /etc/mail/relay-domains file. This is where your day can be either short or long, depending on who you plan on sending mail to.
So rather than listing the 1000+ domains I send my newsletter to, I found something even easier. Instead of listing "gmail.com", Sendmail will also accept ".com". So in short, you can simply put the top-level domain extensions like ".net" or ".org" (without quotes). So if you're getting "550 Relaying denied" errors, it could be that you simply need to add the most popular domain extensions to your relay-domains file.
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