Support intro
Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): 27.0.2
Operating system and version (eg, Ubuntu 20.04): replace me
Apache or nginx version (eg, Apache 2.4.25): replace me
PHP version (eg, 7.4): replace me
The issue you are facing:
Can’t get SMTP to work with mailbox.org.
Symfony\Component\Mailer\Exception\TransportException: Failed to authenticate on SMTP server with username "xxx@mailbox.org" using the following authenticators: "LOGIN", "PLAIN". Authenticator "LOGIN" returned "Expected response code "235" but got code "535", with message "535 5.7.8 Error: authentication failed: UGFzc3dvcmQ6".". Authenticator "PLAIN" returned "Expected response code "235" but got code "535", with message "535 5.7.8 Error: authentication failed: UGFzc3dvcmQ6".".
Using swaks on same instance I can send the email without any issues:
swaks --to yyy@gmail.com --from xxx@mailbox.org --server smtp.mailbox.org --port 587 --auth-user xxx@mailbox.org --auth-password 'zzz' --tls --header "Subject: Test Email" --body "This is a test email."
Here’s swaks log:
=== Trying smtp.mailbox.org:587...
=== Connected to smtp.mailbox.org.
<- 220 smtp1.mailbox.org ESMTP Postfix
-> EHLO {hidden}
<- 250-smtp1.mailbox.org
<- 250-PIPELINING
<- 250-SIZE 143699726
<- 250-ETRN
<- 250-STARTTLS
<- 250-ENHANCEDSTATUSCODES
<- 250-8BITMIME
<- 250-DSN
<- 250 CHUNKING
-> STARTTLS
<- 220 2.0.0 Ready to start TLS
=== TLS started with cipher TLSv1.3:TLS_AES_256_GCM_SHA384:256
=== TLS no local certificate set
=== TLS peer DN="/CN=*.mailbox.org"
~> EHLO {hidden}
<~ 250-smtp1.mailbox.org
<~ 250-PIPELINING
<~ 250-SIZE 143699726
<~ 250-ETRN
<~ 250-AUTH PLAIN LOGIN
<~ 250-AUTH=PLAIN LOGIN
<~ 250-ENHANCEDSTATUSCODES
<~ 250-8BITMIME
<~ 250-DSN
<~ 250 CHUNKING
~> AUTH LOGIN
<~ 334 VXNlcm5hbWU6
~> {hidden}
<~ 334 UGFzc3dvcmQ6
~> {hidden}
<~ 235 2.7.0 Authentication successful
~> MAIL FROM:<{hidden}@mailbox.org>
<~ 250 2.1.0 Ok
~> RCPT TO:<{hidden}@gmail.com>
<~ 250 2.1.5 Ok
~> DATA
<~ 354 End data with <CR><LF>.<CR><LF>
~> Date: Sat, 02 Sep 2023 21:49:15 +0300
~> To: {hidden}@gmail.com
~> From: {hidden}@mailbox.org
~> Subject: Test Email
~> Message-Id: <20230902214915.149817@{hidden}>
~> X-Mailer: swaks vDEVRELEASE jetmore.org/john/code/swaks/
~>
~> This is a test email.
~>
~>
~> .
<~ 250 2.0.0 Ok: queued as AC21E1A57
~> QUIT
<~ 221 2.0.0 Bye
=== Connection closed with remote host.
Here’s the config.php in nextcloud:
'mail_smtpmode' => 'smtp',
'mail_smtphost' => 'smtp.mailbox.org',
'mail_smtpport' => '587',
'mail_smtpsecure' => 'tls',
'mail_smtpauth' => true,
'mail_smtpauthtype' => 'LOGIN',
'mail_smtpname' => 'xxx@mailbox.org',
'mail_from_address' => 'xxx',
'mail_domain' => 'mailbox.org',
'mail_smtppassword' => 'zzz',
Is this the first time you’ve seen this error? (Y/N): N
Steps to replicate it:
- Paste configuration from previous section (replace xxx with email and zzz with your password)
- Click on Send Email
The output of your Nextcloud log in Admin > Logging:
PASTE HERE
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
PASTE HERE
The output of your Apache/nginx/system log in /var/log/____:
Symfony\Component\Mailer\Exception\TransportException: Failed to authenticate on SMTP server with username "xxx@mailbox.org" using the following authenticators: "LOGIN", "PLAIN". Authenticator "LOGIN" returned "Expected response code "235" but got code "535", with message "535 5.7.8 Error: authentication failed: UGFzc3dvcmQ6".". Authenticator "PLAIN" returned "Expected response code "235" but got code "535", with message "535 5.7.8 Error: authentication failed: UGFzc3dvcmQ6".".
