Feature: Recipient allowlist for SMTP relay configuration

[gliwka]

Implements the recipient allowlist discussed in #108

I've kept the UI part simple for now by just displaying the configured allowlist whenever it's active and denying the release on the backend. I could not reuse the regex on the frontend since golang and javascript have a different regex syntax:

[axllent]

This is a really great effort, so thank you so much, I love it! I think the way you've handled the UI error is about as good as it is going to get - we need API errors anyway, and I can't see how to integrate this into JS for the same reason you pointed out (different syntax).

There was a minor debug issue whereby a "rejected" email still appears to have been sent simply because Send() did not return an error when there were no valid recipients (despite nothing being sent), eg:

DEBU[2023/05/05 17:02:43] [smtp] not allowed to relay to example@gmail.com: does not match the allowlist @example\.com$ 
DEBU[2023/05/05 17:02:43] [smtp] relayed message from ralph@ralph via mail.local:25

... so I've changed Send() to return an error instead of nil if len(recipients) == 0, and changed the relay code in mailHandler to show a warning if Send() "fails" (rather than an error).

One last question: I'm not sure how to best document the required yaml regex syntax in the documentation (wiki) in a way that users will understand. Do you have any suggestions?

[gliwka]

@axllent

Thanks for the quick merge :-)

Good that you caught the bug on sending without recipients.

Documentation wise I would make an example using a literal-style YAML string checking for a single hostname, since this will be a common use case. The literal string will make it easier, since you don’t have to escape backslashes.

recipient-whitelist: '@example\.org$'

You can link to this regex playground making it easier for users to find the right one: https://regex101.com/r/mfbicW/1

In which release will this feature land? What’s your release cadence?

[axllent]

Thanks for the documentation suggestions, and the tip about linking to regex101 is a good one (I hadn't thought of that).

As for releases - this should be in the next release which will probably happen within the next 24-48 hours. I prefer to release often (when there are changes of course) rather than build up lots of changes. Mailpit was more-or-less feature-complete for my needs about 6 months ago, but I have been adding more and more features for other users like yourself, so most releases add these new small features :-)