It could be an error from rewriting the address... What have you done ?
I would try this in the squirrelmail.conf or what ever other file you have used with the alias:
Code:
Alias /webmail /opt/squirrelmail
Alias /sm /opt/squirrelmail
<Directory /opt/squirrelmail>
Options +FollowSymLinks
Order allow,deny
allow from all
RewriteEngine on
RewriteCond %{HTTPS} !=on
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
</Directory>