[enhancement]: Configurable MX Record Host

[s-aga-r]

Which feature or improvement would you like to request?

Stalwart currently generates the following MX record:

MX    example.com    10 a1-blr.mail.emailservice.com.

In production environments, load balancers are commonly used as MX records (e.g., mxa.emailservice.com and mxb.emailservice.com). These load balancers distribute email traffic across multiple inbound SMTP servers, ensuring high availability and zero downtime.

We can have a configuration option in the Network Settings (or a relevant section) to specify a custom FQDN for the MX record. By default, this should fall back to the Stalwart hostname, but users should be able to override it with their desired FQDN.

Is your feature request related to a problem?

No response

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