I honestly think that private email solicitation of players that are already in an existing fellowship should not be accepted and any player that does this should be banned. It is not only disruptive to the team but disrespectful and annoying. Specially when it's an Ambassador or Archmage that is doing the soliciting by way of flattery and deception. If a player wishes to leave and look for a different fellowship by their own accord then that should be respected but for someone else knowing that the player is in an active fellowship and still seeking to disrupt that fellowship with such a deceptive and sneaky act should be banned from the game. It takes a lot of hard work and hours of dedication in a fellowship to to become a family and team that is helping each other push forward and grow. So many events that the team prepares for to be able to make a fellowship function properly. No one needs the annoyance of others coming in with deception to disruptive all the hard work the team has put in. Not Acceptable.
An archmage doesn't have a right to put blinders on their members and make sure they never see offers from better fellowships. They don't own their players. Players are free to come and go as they choose. They are also free to receive messages and send messages from anyone. If they don't wish to receive a message, they can ignore it. Most of the time, fellowship solicitations are obvious in the title of the message. They can simply click that little "x" and delete it without any hassle at all. Problem solved.
What an archmage should do if they are losing players to these messages, is to build a better fellowship! If you don't know how, then ask other archmages here; we will be happy to give you some tips.
Players who are happy CANNOT be poached. No archmage has a right to keep unhappy players in your fellowship. Those players will eventually leave anyway, so your job as an archmage, is to build a team of happy players who want to be there.
Now, if a player is getting multiple requests from the same fellowship, they may get annoyed by that. They could always "report" the message and say they are being harrassed with too many messages from the same source. Inno may ban them from contacting that player again. But I suspect you mind these offers a whole lot more than the players do.
That said, there are some awful fellowships out there. "Rescuing" players from a seriously bad fellowship is not a bad thing. I have one player who tells me all the time that he would have left Elvenar by now if I hadn't rescued him. He thanks me at least once a month. He just never knew what a "good" fellowship was until he came to mine... and yes, I rescued him from a terrible fellowship. I never "poach" players from good fellowships ... never. If you have a good fellowship, your players are safe from me. But if you have a bad fellowship, where half your players have black triangles next to their names on Elvenstats, and if you have one player who is doing 2K in the tourney and your group is averaging 3 chests, then heck yeah, I'm going to rescue that player. I'm not going to let Elvenar lose a good player because a bad archmage is too lazy or clueless to do their job. (Note I am NOT saying that about you ... I'm just saying that in general.)
The bottom line is ... no one owns players. Not the fellowship .... and not the one who is trying to recruit.