To do this you will need to change some code in Squirrelmail.
The page that does the login, from that you can with PHP look at the domain name and then make sure that only current domain is after the @. If you are not a PHP developer it could take some time to find out how to do this, and I do not really recommend this solution.
The thing you should do instead is to create one webmail that all users log in from and then SSL encrypt the page, to make sure that users do not send their e-mail credentials via clear text, and then redirect domain.com/webmail to e.g. https://ssl.yoursite.com/webmail
I have built several shared hosting environments hosting thousands of clients, but none of them are dedicated per-domain. All of them are placed on a secure location (simply because I refuse to build webmail access without SSL).
Now the reason why this should be the approach is, that users usually access their webmail from everywhere, which drastically increases the risk of someone sniffing network traffic, thus gaining access.
Nor would you like this to happen, because if that happens your server could be used as a source of unsolicited bulk e-mail (SPAM).
Best Answer
Install the Squirrel Logger plug-in, and specify the log file, and what you want logged. Make sure you configure the config.php file before enabling the plug-in.
By default, Squirrel Mail has no logs of it's own, you'd have to read/interpret the IMAP and LDAP logs. Installing this quick plug-in allows Squirrel Mail to write a log of it's own, and you can configure what gets logged there. Don't forget to add the log to your log rotation program so it doesn't take over the disk.