I have had a similar problem with the 4 letter extension causing issues. What is really annoying is the sites that allow the signup with the 4 letters but then

I have had a similar problem with the 4 letter extension causing issues. What is really annoying is the sites that allow the signup with the 4 letters but then proceed to fail at every other opportunity up to and including opt out(GAP). I spent an hour with them over the phone before anyone even understood what I was saying. I am also now using Fastmail addresses for anything I don't have some trust for and isn't a problem if lost. I no longer use my domain for things outside of this because it seems like an obvious step to simply link an unknown domain together, even if a laughable amount of companies don't.