We need a class, that tries to send a probe message to a given address.
Depending on the answeres of the other SMTP server, we can deduce if the address exists or not.
We then may implement two mailets:
- one verifying the recipients of incoming emails
- one verifying its sender
Here is the page for POSTFIX describing this issue.
Some various concerns:
- Avoid probing locally handled domains
- Implement a address cache: do we need persistence for it? POSTFIX does it. We may introduce a corresponding storage API in data-api (Map<MailAddress, IsValid>, basically). Note that cache expiration differs between existing and nonexisting entries.
Idea from @whiteadam proposed on gitter