We need to set up a server-wide redirect rule in httpd/conf.d but I'm not sure of the exact syntax. There are hundreds of domains, a mixture of .com, .co.uk, .net etc.
Basically, anybody accessing domain.tld/webmail should be redirected to webmail.domain.tld.
The reason for this is a change of hosting provider. The old host supplied email via /webmail, whereas the new host uses webmail.domain.com.
A 301 redirect would be ideal, but it must be server-wide, not htaccess.