[enhancement]: Hope the outbound in the SMTP protocol can support MTA Hooks

[Boomboomdunce]

Which feature or improvement would you like to request?

Hope that the outbound in the SMTP protocol can support Mta Hooks, so that it can be applied to:

1. When the mail-server acts as a relay mail server, mail checks can be performed through Mta Hooks in the outbound direction, and malicious/spam emails can be prevented from being delivered. This method can be applied to cloud mail (such as Office365).

2. When the mail-server acts as a mail server, Mta Hooks in the outbound direction can be used to address non-compliant or sensitive email outgoing detection, to avoid employees inadvertently leaking information when using email.

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