When sending from Roundcube on an Ubuntu server with plesk, we get the following error:

SMTP Error (535): Authentication failed

Symptoms

While trying to send an email via Roundcube, the following error appears:

SMTP Error (535): Authentication failed

In /var/log/maillog following entries can be found:

warning: SASL authentication failure: cannot connect to saslauthd server: No such file or directory
warning: SASL authentication failure: Password verification failed
warning: localhost[::1]: SASL PLAIN authentication failed: generic failure

saslauthd service is running:

    service saslauthd status
    Redirecting to /bin/systemctl status saslauthd.service
    ● saslauthd.service - SASL authentication daemon.
    Loaded: loaded (/usr/lib/systemd/system/saslauthd.service; enabled; vendor preset: disabled)
    Active: active (running)

compatibility_level value was previously defined in /etc/postfix/main.cf as following:

# grep compatibility_level /etc/postfix/main.cf
compatibility_level = 2

Here is the fix: Comment out compatibility_level in /etc/postfix/main.cf

Resolution
Login to the server via SSH.
Create a backup of /etc/postfix/main.cf:

# cp -p /etc/postfix/main.cf /etc/postfix/main.cf_bck

Remove compatibility_level value from /etc/postfix/main.cf using the favorite text editor.

# nano /etc/postfix/main.cf
#compatibility_level = 2

Check SMTP daemon configuration:
If RHEL/CentOS/CloudLinux is used, verify that chrooted mode is disabled:

egrep "^smtp " /etc/postfix/master.cf | grep inet
smtp inet n - n - - smtpd

If Debian/Ubuntu is used, verify that chrooted mode is enabled:

egrep "^smtp " /etc/postfix/master.cf | grep inet
smtp inet n - y - - smtpd

Reload postfix configuration:

service postfix reload

Source: https://support.plesk.com/hc/en-us/articles/360004930633-Unable-to-send-an-email-from-Plesk-server-SMTP-Error-535-Authentication-failed