postfix stopped working after fail2ban

Since yesterday my Postfix stopped working: It doesn't receive any e-mails anymore (from other email addresses, not my own). There's nothing in mail.log or mail.err. Yesterday I installed fail2ban on my Ubuntu, did update & upgrade, and installed sendmail with fail2ban, which was an error, I removed sendmail again. But now I just can't get postfix to work. When I get its status it says

root@vps219158:/etc/init.d# sudo systemctl status postfix.service
* postfix.service
Loaded: masked (/dev/null; bad)
Active: inactive (dead)

And when I try to restart it

Failed to restart postfix.service: Unit postfix.service is masked.

I don't understand why it is masked.

Fail2ban is deactivated, I also checked the iptables: Nothing is banned. When I send an email to my email account from any one of my 3 other email accounts (@hotmail.com etc.) it doesn't report that the mails couldn't be sent - they just don't appear in my inbox (i.e. not on the server, "mail" command shows no new mails).

What more could I test? What could possibly lead to this issue?

EDIT: Status is: From outside, the SMTP connection fails. Postfix says it's running. Nothing in It ran until yesterday. On localhost, I can telnet to "localhost 25" and send a mail, but it doesn't get to my inbox (same for ehlo mydomain.com and ehlo localhost).

Thanks in advance for some help!

1 Answer

Ok, that was tough:

• I reinstalled postfix (apt-get postfix)
• I ran newaliases because some aliases were missing
• I had to reboot because port 25 was still in use even though I had uninstalled sendmail
• I had to unmask the postfix service

No clue why all of this lead to the solution, I couldn't identify the source of the problem. But apparently, now it works.