SMTP Authentication Failed (535) - IMAP works fine, Thunderbird works fine

[Segelwolf]

Hi everyone,
I'm experiencing an SMTP authentication issue with EspoCRM 9.1.8 (self-hosted on ALL-INKL.com server) and would appreciate your help.
What works:

• ✅ IMAP receives emails perfectly with these credentials
• ✅ Thunderbird sends emails via SMTP with the exact same credentials, webmail@provider also works fine with the same credentials

What doesn't work:

• ❌ EspoCRM SMTP authentication fails

Configuration:
Email: wolf@segelwolf.com
SMTP Server: smtp.all-inkl.com
Port: 465
Security: SSL/TLS
Authentication: Yes
Username: wolf@segelwolf.com
Password: [same as IMAP, works in Thunderbird]Error message:
ERROR: (535) Email sending error: Failed to authenticate on SMTP server
with username "wolf@segelwolf.com" using the following authenticators: "PLAIN".
Authenticator "PLAIN" returned "Expected response code "235" but got code "535",
with message "535 5.7.8 Error: authentication failed: (reason unavailable)".Environment:

• EspoCRM 9.1.8
• Server: w020ba72.kasserver.com (ALL-INKL.com)
• IMAP and SMTP server are from the same provider (ALL-INKL)

What I've tried:

• Triple-checked credentials (copy-paste from working Thunderbird config)
• Tested both ports 465 (SSL) and 587 (STARTTLS)
• Verified server settings with hosting provider documentation

Since IMAP and Thunderbird work perfectly with the same credentials, I suspect there might be something specific about how EspoCRM handles SMTP authentication vs. how Thunderbird does it.
Has anyone experienced similar issues with ALL-INKL.com or other hosting providers? Any suggestions would be greatly appreciated!
Thanks in advance for your help!

[shalmaxb]

Did you send the testmail? Do you have the correct credentials in system SMTP and in your personal account (user preferences)? Anything i n your log file?

[Segelwolf]

IMAP Testmail ok, SMTP Testmail error 535 as quoted

[Segelwolf]

I cannot send a testmail, getting a 500 error. The error msg above is from the logfile

[ThomasB]

Have you tried with login instead of plain auth? All-inkl email account usually works with EspoCRM.

[Segelwolf]

Login doesn't work either, always getting wrong password/username: Still: both pw and user are exactly the same as for webmail and thunderbird.

[dimyy]

Plain auth don't supported by the symfony yet. We patch code for plain atuh. It's symfony EsmtpTransport.php problem. You can try this patch:

[Segelwolf]

tried your suggestion: did not work, still error 500, credentials not valid

[Segelwolf]

The whole thing is bloody useless without the possibility to send Emails.

[shalmaxb]

please, send your logfile

[Segelwolf]

[2025-11-23 09:50:16] ERROR: (535) Email sending error: Failed to authenticate on SMTP server with username "wolf@segelwolf.com" using the following authenticators: "LOGIN". Authenticator "LOGIN" returned "Expected response code "235" but got code "535", with message "535 5.7.8 Error: authentication failed: (reason unavailable)".". :: /www/htdocs/w020ba72/crm.segelwolf.com/vendor/symfony/mailer/Transport/Smtp/EsmtpTransport.php(226)
[2025-11-23 09:50:16] WARNING: Email sending:invalidCredentials; 0
[2025-11-23 13:28:24] ERROR: (535) Email sending error: Failed to authenticate on SMTP server with username "wolf@segelwolf.com" using the following authenticators: "LOGIN". Authenticator "LOGIN" returned "Expected response code "235" but got code "535", with message "535 5.7.8 Error: authentication failed: (reason unavailable)".". :: /www/htdocs/w020ba72/crm.segelwolf.com/vendor/symfony/mailer/Transport/Smtp/EsmtpTransport.php(229)
[2025-11-23 13:28:24] WARNING: Email sending:invalidCredentials; 0
[2025-11-23 13:30:02] ERROR: (535) Email sending error: Failed to authenticate on SMTP server with username "wolf@segelwolf.com" using the following authenticators: "LOGIN". Authenticator "LOGIN" returned "Expected response code "235" but got code "535", with message "535 5.7.8 Error: authentication failed: (reason unavailable)".". :: /www/htdocs/w020ba72/crm.segelwolf.com/vendor/symfony/mailer/Transport/Smtp/EsmtpTransport.php(229)
[2025-11-23 13:30:02] WARNING: Email sending:invalidCredentials; 0
[2025-11-23 13:32:38] ERROR: (535) Email sending error: Failed to authenticate on SMTP server with username "Wolf Knipfer" using the following authenticators: "LOGIN". Authenticator "LOGIN" returned "Expected response code "334" but got code "535", with message "535 5.7.8 Error: authentication failed: (reason unavailable)".". :: /www/htdocs/w020ba72/crm.segelwolf.com/vendor/symfony/mailer/Transport/Smtp/EsmtpTransport.php(229)
[2025-11-23 13:32:38] WARNING: Email sending:invalidCredentials; 0
[2025-11-23 13:32:59] ERROR: (535) Email sending error: Failed to authenticate on SMTP server with username "Wolf" using the following authenticators: "LOGIN". Authenticator "LOGIN" returned "Expected response code "235" but got code "535", with message "535 5.7.8 Error: authentication failed: (reason unavailable)".". :: /www/htdocs/w020ba72/crm.segelwolf.com/vendor/symfony/mailer/Transport/Smtp/EsmtpTransport.php(229)
[2025-11-23 13:32:59] WARNING: Email sending:invalidCredentials; 0

[ThomasB]

All-inkl E-mail accounts do work with EspoCRM, so it must be an issue with your host or installation. Maybe some ports are blocked?

[Segelwolf]

as I said, the login data and ports work fine with webmail and thunderbird. It is only EspoCEM that makes SMTP problems (by the way: IMAP works fine in EspoCEM with the same credentials)

[shalmaxb]

Does your password contain special characters?
All-Inkl creates beside the email address also a "name" for the email account like so: "m0592pc5c". It maybe, that this is expected in the login.
Also did you try another port?

[Segelwolf]

1. PW is 20 digits and contains only Letters and numbers
2. Of course I also tried the name instead of the Email address - same result
3. Which port should I try! As far as I know the port is defined by the SMTP security: Either 465 for SSL/TLS or 587 for Starttls. I tried both without success.

[ThomasB]

This is what works with my system and all-ink email.
Group Account Setup

Use SMTP: yes
Host: w020ba72.kasserver.com
Security STARTTLS
Port: 587
Auth-mechanism: LOGIN
Username: email address or name like m0592pc5c
Password: your password

Maybe try change your password to a simple one to test. Otherwise what is the OS you use on your server? Maybe ask ALL-Inkl for support because it seems more a configuration issue of your server.