This page is reached from Services → Webmail Server, then the Advanced tab.

Here you can set various options for the VPOP3 Webmail/settings service.

If you check this option, then you can specify the user's password in a URL to Webmail

For instance, if the VPOP3 server is on 192.168.1.1, then the Webmail address would normally be

http://192.168.1.1:5108

If the Allow Password in URL option is checked, then you can go to the address like:

http://192.168.1.1:5108?user=postmaster&password=admin

to go to the Webmail and automatically log in using the specified details

For a bit more security you could use:

http://192.168.1.1:5108?user=postmaster&md5pass=MD5-hash-of-password

In this case, you specify the MD5 hash of the password. That makes it more difficult to find the original password, but does not prevent a replay attack using the details.

Generally, we don't recommend using this option, because of the security implications, but in some cases it can be useful.