Resolved SMTP fails on all new email addresses since Plesk update

[KirkM]

Server operating system version

AlmaLinux 8

Plesk version and microupdate number

Plesk 18.0.75

When I test smtp on new email addresses on any domain on my server with PHPMailer smtp test script and v7.0.1, I get

SMTP ERROR: Username command failed: 535 5.7.8 Error: authentication failed: authentication failure SMTP error: Authentication failed: Username command failed

Existing domains before the update are fine when do the exact same test but any new ones always fail with the identical setups on the same server and even the same domain.

 

[KirkM]

A bit more detail -
Both the successful tests with the old domains and the failures with the new domains settings are:

Username: [email protected]
Password: {password}
TLS, Port 587
Postfix / Dovecot

I have tried:
opening all ports on the firewall
Disabling the firewall
uninstalling the firewall extension
Reinstalling the firewall extension and setting all ports open
Flushing iptables
switching from Postfix to Qmail and then back to rebuild Postfix
Reinstalled the AlmaLinux kernel
Ran all of the plesk repair commands and completely rebuilt everything
Ran plesk repair installation to do a bootstrapper repair of Plesk

I set up the new email addresses in an email client and they would not connect until I reinstalled Postfix. And even though they eventually connected to the SMTP server, it took a LOOOOONNNNG time and it is very slow to connect each time mail is fetched. They completely fail when I run an SMTP test with PHPMailer. Old domains, no problem at all.

The email addresses that have existed for a while are just fine. Doesn't matter which domain it is. This behavior is server-wide. Something has been broken in the psa SMTP mail authentication but I don't know where else to look.

 

[KirkM]

Full error:

2026-01-12 00:42:10 Connection: opening to mail.xxxxxxxx.com:587, timeout=30, options=array() 2026-01-12 00:42:10 Connection: opened 2026-01-12 00:42:10 SERVER -> CLIENT: 220 secure.xxxxxxxx.com ESMTP Postfix
Connected!

2026-01-12 00:42:10 CLIENT -> SERVER: EHLO secure.xxxxxxxx.com 2026-01-12 00:42:10 SERVER -> CLIENT: 250-secure.xxxxxxx.com 250-PIPELINING 250-SIZE 51200000 250-ETRN 250-STARTTLS 250-ENHANCEDSTATUSCODES 250-8BITMIME 250-DSN 250 CHUNKING
EHLO SUCESSFUL!

2026-01-12 00:42:10 CLIENT -> SERVER: STARTTLS 2026-01-12 00:42:10 SERVER -> CLIENT: 220 2.0.0 Ready to start TLS
ENCRYPTION STARTED SUCCESSFULLY!

2026-01-12 00:42:10 CLIENT -> SERVER: EHLO secure.xxxxxxxxx.com 2026-01-12 00:42:10 SERVER -> CLIENT: 250-secure.xxxxxxxx.com 250-PIPELINING 250-SIZE 51200000 250-ETRN 250-AUTH DIGEST-MD5 CRAM-MD5 PLAIN LOGIN 250-ENHANCEDSTATUSCODES 250-8BITMIME 250-DSN 250 CHUNKING
SMTP AUTH SUCCESSFUL!

2026-01-12 00:42:10 CLIENT -> SERVER: AUTH CRAM-MD5 2026-01-12 00:42:10 SERVER -> CLIENT: 334 PDMxNTQ5MTM2MjIuNjU3MDk1NUBzZWN1cmUubWRzZXJ2ZXIxLmNvbT4= 2026-01-12 00:42:10 CLIENT -> SERVER: [credentials hidden] 2026-01-12 00:42:10 SERVER -> CLIENT: 535 5.7.8 Error: authentication failed: authentication failure 2026-01-12 00:42:10 SMTP ERROR: Username command failed: 535 5.7.8 Error: authentication failed: authentication failure
SMTP error:
Authentication failed: Username command failed 2026-01-12 00:42:10 CLIENT -> SERVER: QUIT 2026-01-12 00:42:10 SERVER -> CLIENT: 221 2.0.0 Bye 2026-01-12 00:42:10 Connection: closed

And yes, the username and passwords are correct.

 

[ChristophRo]

check "Tools & Settings" --> "Security Policy" --> "Storing email passwords" and make sure it's set to "Symmetric encryption"

 

[KirkM]

It was set to hashing. Changing it to Symmetric encryption did the trick. Thank you so much!

 

[KirkM]

They have disabled the ability to mark as resolved, but it is resolved!