Sound like a reasonable thing to do. Might be worth following up with dovecot upstream anyway?

Do you mean updating the 17.09 package? I think we shouldn't update from 2.2.33.2 to 2.3.0.
2.2.33.2 is the newest release in the 2.2 branch so we should be fine.

I did backport this to 17.09 and then realized that we can not use strMatching there yet. Do you have another solution? E.g. build another type

The issue is that the regex code isn't present on 17.09 per default.

I meant asking the upstream project how one would properly escape an "

Well the other solution would be to use stringToCharacters and check for " using elem. But that's probably horribly inefficient.

I asked them about escaping on IRC.

I did backport the "faulty" commit from in #34629, so it either needs to be fixed in 17.09 or reverted.

It is not faulty. Mailbox names with quotes inside do lead to doveconf complaining about errors in the configuration file after and before that commit.
This PR would just make it impossible to specify a mailbox qith quotes in the name. So essentially, right now Dovecot is complaining whereas if this gets merged nixos-rebuild will complain about it.