Yes, I just fixed your problem. I may edit the update / upgrade script in the future, but this problem happens only occasionally. I have run into it before (doesn't usually happen), and only on a few servers running EHCP.
If you run into a problem like this, the first thing to do is disable ipv6 support in postfix like so:
sudo cp /etc/postfix/main.cf /etc/postfix/main.cf.bkup
sudo nano /etc/postfix/main.cf
Find:
inet_interfaces = (ANY_SETTING)
Replace with:
inet_interfaces = ipv4
Save and exit.
Then, run the Squirrel Mail configuration utility.
/var/www/new/ehcp/webmail/config/conf.pl
Enter 2 to edit the server settings.
Enter 3 and change the setting to Sendmail.
Enter 1 to change your domain name for the server.
Enter S to save changes.
Enter Q to quit.
I tested sending an email from RoundCube and SquirrelMail. Doing the above fixed your issue.