Suppose Alice and Bob together own the domain surname.com, and want to use email addresses alice@surname.com and bob@surname.com, respectively. They also want to use different email services, say, mailfence for Alice and startmail for Bob, as Alice uses alice@mailfence.com and bob uses bob@startmail.com.
In their DNS provider (or designated similar service, e.g. cloudflare), they could set forwarding rules so that alice@surname.com forwards to alice@mailfence.com, and bob@surname.com forwards to bob@startmail.com.
This will work for incoming email, but a problem remains for outgoing mail: the messages they send will still appear to come from the addresses alice@mailfence.com and bob@startmail.com instead of alice@surname.com and bob@surname.com.
To send from custom-domain addresses, mailfence and startmail each support setting provider-specific MX records on the domain, which would enable that email provider to send as well as receive from the custom domain. However, setting MX records that forward to mailfence will prevent mail from being sent to startmail, and vice-versa; contradictory MX records would be required. So it seems this solution does not work for Alice and Bob to use different email providers.
Is there a solution that enables different accounts from different email providers to both receive and send from the same custom domain?
