I would just give our standard caution:
Genuine recommendations by PM only (private message) are acceptable - but beware of the possibility that someone who appears to be offering a recommendation may in reality be engaged in self-promotion! Please let us know if you think anyone is abusing the system by soliciting for business or 'spamming'.
Always treat recommendations from unknown persons with caution. However, there are many long standing members (check the date of joining and number of posts) who make a valuable contribution to the forum. If you receive a recommendation from one of them, it could be worth considering seriously (while still exercising a degree of caution).
If you receive a recommendation from anyone else, please exercise extreme caution!