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.

[shalmaxb]

In your other post I read, that you are using Starlink. You cannot send emails configured in other providers, like AllInkl. via Starlink. Search for that, it is a known problem.
By the way, AllInkl. in my opinion is not a good choice.

[Segelwolf]

If that is true, so why does Thunderbird work perfectly via StarLink, so does all-inkl webmail.

[ThomasB]

Found this post: https://www.purevpn.de/blog/so-leite...arlink-weiter/

Which ports are blocked by Starlink?

Starlink routers typically block ports used for sending and receiving email, as these are known to be used for sending spam and causing security issues. The ports blocked by Starlink are listed below:

SMTP port 25
POP3 port 110
SMTP over SSL port 465
SMTP relay port 587
IMAP port 143

Here I only find port 25 and 465 in Indonesia.

https://starlink.com/support/article/c3caacdf-1c1f-98db-b821-bbb36ca9d89b

[Segelwolf]

These ports are not blocked. And the StarLink router is configured to pass through to a MikroTik router that does the work