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Debian 11 is approaching its end-of-life (vendor EOL date - August 31, 2026). Plesk Obsidian 18.0.80 will be the last release to support it.
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I've got the same problem. In my /var/log/mailog/ I have repeated lines about not being able to start Postfix (which the PHP mail() function uses). For example:
Nov 6 21:20:46 vero-solutions postfix/postfix-script[61870]: starting the Postfix mail system
Nov 6 21:20:46 vero-solutions postfix/master[61872]: fatal: open lock file /var/lib/postfix/master.lock: unable to set exclusive lock: Resource temporarily unavailable
Nov 6 21:20:47 vero-solutions postfix/master[61871]: fatal: daemon initialization failure
Nov 6 21:20:48 vero-solutions postfix/postfix-script[61876]: fatal: mail system startup failed
Here are the results:
ls -als /usr/sbin/sendmail 0 lrwxrwxrwx 1 root root 21 Nov 7 20:12 /usr/sbin/sendmail -> /etc/alternatives/mta
ls -als /etc/alternatives/mta
0 lrwxrwxrwx 1 root root 26 Nov 7 20:12 /etc/alternatives/mta -> /usr/sbin/sendmail.postfix
alternatives --display mta
mta - status is auto.
link currently points to /usr/sbin/sendmail.postfix
/usr/sbin/sendmail.postfix - priority 30
slave mta-pam: /etc/postfix/pam.d/smtp.postfix
slave mta-mailq: /usr/bin/mailq.postfix
slave mta-newaliases: /usr/bin/newaliases.postfix
slave mta-rmail: /usr/bin/rmail.postfix
slave mta-sendmail: /usr/lib/sendmail.postfix
slave mta-mailqman: /usr/share/man/man1/mailq.postfix.1.gz
slave mta-newaliasesman: /usr/share/man/man1/newaliases.postfix.1.gz
slave mta-aliasesman: /usr/share/man/man5/aliases.postfix.5.gz
slave mta-sendmailman: /usr/share/man/man1/sendmail.postfix.1.gz
Current `best' version is /usr/sbin/sendmail.postfix.
ls -als /usr/sbin/sendmail.postfix
ls -als /usr/lib/sendmail.postfix
Anyway, while I don't have a test server handy right now, this should solve your issue (all the lines must be entered one after another before you'll see any response):
I've got another identical server which has updated to Plesk Obsidian, and the PHP mail() function is still working there. For comparison, the output of alternatives --display mtaon this working server is:
mta - status is manual.
link currently points to /usr/lib64/plesk-9.0/postfix-sendmail-wrapper
/usr/sbin/sendmail.postfix - priority 30
slave mta-pam: /etc/pam.d/smtp.postfix
slave mta-mailq: /usr/bin/mailq.postfix
slave mta-newaliases: /usr/bin/newaliases.postfix
slave mta-rmail: /usr/bin/rmail.postfix
slave mta-sendmail: /usr/lib/sendmail.postfix
slave mta-mailqman: /usr/share/man/man1/mailq.postfix.1.gz
slave mta-newaliasesman: /usr/share/man/man1/newaliases.postfix.1.gz
slave mta-aliasesman: /usr/share/man/man5/aliases.postfix.5.gz
slave mta-sendmailman: /usr/share/man/man1/sendmail.postfix.1.gz
/usr/lib64/plesk-9.0/postfix-sendmail-wrapper - priority 90
slave mta-pam: /etc/pam.d/smtp.postfix
slave mta-mailq: /usr/bin/mailq.postfix
slave mta-newaliases: /usr/bin/newaliases.postfix
slave mta-rmail: /usr/bin/rmail.postfix
slave mta-sendmail: /usr/lib64/plesk-9.0/postfix-sendmail-wrapper
slave mta-mailqman: /usr/share/man/man1/mailq.postfix.1.gz
slave mta-newaliasesman: /usr/share/man/man1/newaliases.postfix.1.gz
slave mta-aliasesman: /usr/share/man/man5/aliases.postfix.5.gz
slave mta-sendmailman: /usr/share/man/man1/sendmail.postfix.1.gz
Current `best' version is /usr/lib64/plesk-9.0/postfix-sendmail-wrapper.
I removed the --slave /etc/pam.d/smtp mta-pam /etc/pam.d/smtp.postfix \ line from the multiline command and re-ran it and there was no link message, and the PHP mail() command is now working. The output of alternatives --display mta is now:
mta - status is auto.
link currently points to /usr/lib64/plesk-9.0/postfix-sendmail-wrapper
/usr/sbin/sendmail.postfix - priority 30
slave mta-pam: /etc/postfix/pam.d/smtp.postfix
slave mta-mailq: /usr/bin/mailq.postfix
slave mta-newaliases: /usr/bin/newaliases.postfix
slave mta-rmail: /usr/bin/rmail.postfix
slave mta-sendmail: /usr/lib/sendmail.postfix
slave mta-mailqman: /usr/share/man/man1/mailq.postfix.1.gz
slave mta-newaliasesman: /usr/share/man/man1/newaliases.postfix.1.gz
slave mta-aliasesman: /usr/share/man/man5/aliases.postfix.5.gz
slave mta-sendmailman: /usr/share/man/man1/sendmail.postfix.1.gz
/usr/lib64/plesk-9.0/postfix-sendmail-wrapper - priority 90 slave mta-pam: (null)
slave mta-mailq: /usr/bin/mailq.postfix
slave mta-newaliases: /usr/bin/newaliases.postfix
slave mta-rmail: /usr/bin/rmail.postfix
slave mta-sendmail: /usr/lib/sendmail.postfix
slave mta-mailqman: /usr/share/man/man1/mailq.postfix.1.gz
slave mta-newaliasesman: /usr/share/man/man1/newaliases.postfix.1.gz
slave mta-aliasesman: /usr/share/man/man5/aliases.postfix.5.gz
slave mta-sendmailman: /usr/share/man/man1/sendmail.postfix.1.gz
Current `best' version is /usr/lib64/plesk-9.0/postfix-sendmail-wrapper.
I don't know if that mta-pam line is important and needs fixing, but it's working with it being (null).