How to not forward plus addressing to an alias?

[p3k]

Consider an alias like mail@domain.tld → foo@gmail.com, bar@ymail.com.

Sending a message to mail+test@domain.tld results in the recipient addresses foo+test@gmail.com and bar+test@ymail.com.

The message to Gmail is received. The one to Yahoo Mail is not – because Yahoo does not support plus addressing:

<bar+test@domain.tld>: host mta6.am0.yahoodns.net[67.195.204.79]
    said: 554 Message not accepted. From header is invalid or contains
    unacceptable characters. See
    https://senders.yahooinc.com/smtp-error-codes#other-policy-errors (in reply
    to end of DATA command)

I made sure the Yahoo Mail address causes the error by directly sending a message to bar+test@ymail.com.

So I would need to prevent the plus addressing being added to the bar@ymail.com address.

I already tried it with a regex: /mail(?:\+.*)?/i@domain.tld without success.

Any ideas or suggestions?

[jmrplens]

There is no configuration that does this today, and the reason your regex could not work is that the tag is re-attached after alias resolution, not during it.

In helpers/get-forwarding-addresses.js the whole thing is decided at the end of getForwardingAddresses. If the inbound address has no + it returns the forwarding addresses untouched. If it does, it maps over every one of them:

if (!address.includes('+')) return { ..., addresses: forwardingAddresses };

return {
  ...,
  addresses: forwardingAddresses.map((forwardingAddress) => {
    if (isFQDN(forwardingAddress) || isIP(forwardingAddress) || isURL(forwardingAddress, config.isURLOptions))
      return forwardingAddress;
    return `${parseUsername(forwardingAddress)}+${parseFilter(address)}@${parseHostFromDomainOrAddress(forwardingAddress)}`;
  })
};

That map runs over the destinations your alias already resolved to, so whatever the alias looked like, regex or not, every destination that is an email address gets +test appended. Your regex changed which address was selected; it could not change what happened to it afterwards. The only destinations that come out untouched are the ones caught by that first branch, which are domains, IPs and URLs rather than mailboxes.

Chaining does not get you out of it either, since the second hop would see an inbound address that still contains a + and apply the same transformation again.

I checked for a switch and did not find one: nothing per-alias or per-destination governs this, and the closest existing shape is has_wkd_disabled on the alias model, which is a per-alias opt-out for an unrelated feature. So the fix would be a small feature rather than a setting you have missed, either a per-destination flag or an alias-level "do not propagate subaddressing", and it would slot into exactly that .map().

This is from reading the source rather than running it, so worth confirming with the maintainers, but the branch is explicit enough that I would not expect a configuration answer to exist. Worth raising as a feature request given Yahoo is not the only receiver that rejects a plus in the local part.