Can I redirect the URL
http://mail.example.com into webmail similar to
The "mail" subdomain is usually used to point to your POP/IMAP/SMTP server so that you configure your mail client easily. The "webmail" subdomain is usually points to a different machine that allows your user to check their email with a web browser.
The only way this will work is if you have a web server running on the "mail" machine and you have some control over it. That isn't the case for me. My hosting company doesn't run a web server on their mail machines.
If you have a web server on the "mail" machine, you just need to put a redirect rule into the .htaccess file on that machine.
Whilst this DNS record is present you will struggle to setup the required subdomain/redirect.
You might be able to simply delete this DNS record (if you have access). However, it's probably best to query this with your webhost.