Accessing your domain's mail

sorry couldnt find a better place to post this…
but would anyone here know what are the pros and cons of logging in to your domain’s mail via:

http://yourdomain.com/webmail

and

https://yourdomain.com/webmail

obviously, the second one uses a secure connection (https)… the only problem is i cant log-in using it…

Firefox says: “This connection is untrusted”…
IE says: “Security Alert”…

they complain about the security certificate being not vaild or something…

so i just resort to:

http://yourdomain.com/webmail

but does that pose any security threat??
if so, then how should I do it properly?

i know i can just log-in to cpanel and access the webmail from there… but what if i had a client and i dont want them to access cpanel (since they might mess it up)… i just want them to have access to the mail account…