Currently we keep a DNS rule for every client that has mail.domain.com and this helps us redirect their mail to another server like GMail if they don't want to use our server. The problem is that when these domains change IPs or their individual SSL certs expire they can be locked out of their e-mail or have to confirm security exceptions they don't remove later.
To summarize we have something like this as DNS records:
domain.com 18.104.22.168 mail.domain.com MX 22.214.171.124 <-- sometimes changes for people with GMail
The plan is we move our clients to using a single hostname for connecting to the mail server. So instead all clients would always connect to
mail.secureinternetthing.com - This seems to work fine for new clients, as we tell them the new address to use, but for older clients we want to avoid clients having to change their settings.
If we change the MX record
mail.domain.com to go to the same host as
mail.secureinternetthing.com will our clients reject this?