Suggested feature: have a few mail servers to be configured globally for email sending.
Explanation: let's say, there are two mail relay servers, mail1 and mail2.
At current WHMCS state it is possible only to use mail1 server, for sending invoices, tickets, notifications, whatever else.
If mail1 goes down - there is a some delay between it can be detected, reported as non-operational, and WHMCS is reconfigured to mail2.
What is good to have at WHMCS - ability to specify few mail servers, each with own credentials set, that will be used in round-robin routine, so if one is failed within given timeout, WHMCS will try to use another configured server to send email