There are a variety of methods for this if you are using Apache. You can add an "alias" directive to each domain that points to the common mailserver software:
Code:
<VirtualHost *:80>
ServerName www.domain1.name
Alias /webmail /var/www/webmail
[stuff]
</VirtualHost>
<VirtualHost *:80>
ServerName www.domain2.name
Alias /webmail /var/www/webmail
[stuff]
</VirtualHost>
Handling outbound addressing in this setting can be tricky. In SquirrelMail, for instance, you'll need to use its personalization features so each person sends mail with the proper user@domain. Without this you can end up with everyone having "From: user@yourdomain.com" addresses rather than "From: user@herdomain.com".
If you have some experience with PHP (I'm guessing that is what the webmail software is written in), you could design a custom login page where the person enters user@domain.name and a password. You would then parse the domain from the login address and proceed accordingly.
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