How to install SquirrelMail on ubuntu 18.04

The MX record for mooo.com says that your mail is handled by mail.afraid.org. When I attempt to send email to user1@mooo.com through mail.afraid.org, it says "User unknown in virtual address table".

As for why your outgoing mail is failing, look in your log:

Connections will not be accepted from 183.82.61.111, because the ip is in Spamhaus's list

You seem to be confused as to whether you're sending and receiving mail for mooo.com or for lts.mooo.com. If you want maillts.mooo.com to receive email sent to user1@mooo.com, then you need a MX record for mooo.com that specifies maillts.mooo.com, instead of mail.afraid.org. As it's set up now, it should receive email for user1@lts.mooo.com, but your Postfix configuration doesn't seem to specify that in the mydestination section.

I had exactly the same issue, working with ESXi 6.7 (version 14 VM), and for alternative installers (I needed to use them to access LVM/LUKS full-disk encryption) for both Ubuntu 18.04 and 19.10.

The solution came from following this thread.

You need to toggle boot options with F6, and then add the following parameters:

vga=normal fb=false

Then installation appears to proceed as expected!