meetdilip I admit that I haven't kept up-to-date with this one, so I don't know what's been changed. It still has the beta version labeling, so I'd suggest waiting a little bit more time until all bugs get fixed.
I don't know what do you exactly mean by native mail support. The problem is that the extension modifies the mail drivers in some way/extends part of the mail code in order to send mails, but FreeFlarum does that too already. If you have filled out custom mail driver settings, the language does not break, if you did, everything works just fine. My idea is that FoF Filter is trying to pick up empty mail driver settings (which is FF's default), but fails. Other extensions that send mails, however, are fine, not sure why FoF Filter is an exception there.
We also strongly advise our users to report issues like these to extension authors too, on their own, as we do not have the time to debug or watch bugs in extensions such as these.
If you, however, would like to use the extension soon, I could try creating a FF "patch" fork for the Filter extension that removes the mail alert system, making it compatible with FreeFlarum. Its license should allow this small modification too.
