From a thread I made on this topic

It's simple, give us three options -

1. Receive tells from friends while in dungeons? On by default.
2. Receive tells from party-mates while in dungeons? On by default.
3. Receive tells from anyone on your server while in dungeons. Off by default.
There, problem solved, no matter what your preference. Most people would leave #3 off, because of RMT issues. The only way you get an rmt tell now is if a friend is hacked.