Why not create a white list on the forum for receiving PM?

[lovesmayfamilis]

The forum can add a user to the blacklist. Why not open the same function, only with a whitelist? It would be convenient to receive PM only from people who are on our trust lists. Sometimes it can be very frustrating to receive letters from some people. On the forum, they pretend to be normal, but when it comes to PM, these people turn into real boors.
It is not very convenient to add such characters to the blacklist one by one. It would be much more convenient to receive letters only from those people whom you trust. 

[jackg]

That might seem quite restrictive though? And would you put them in a separate mailbox or just delete them altogether?

If anything constitutes as harrasment you could probably just report their pm to the admin and have it dealg with that way. (I don't think DMs are a good place to have a discussion/argument most people open threads for that stuff).

[Rikafip]

Do you really receive so many unsolicited PMs that something so drastic is needed? And is that hard to just ignore/delete/report them?

I personally don't like your suggestion at all as I often contact people via PM and some of them don't have me on their trust list (and some don't even bother with that feature) meaning that one important channel of communication would disappear for so many people, especially for genuine users who might need help.

[actmyname]

If you have such low signal to noise, then just put up a PGP key. Block all PMs. Make people send encrypted messages to you, or prompt you for temporary correspondence.Presumably, the most likely culprits are Newbie accounts - just block PMs from Newbies.

[RapTarX]

Do you really receive so many unsolicited PMs that something so drastic is needed?

It’s not about receiving so many PMs; check what he referred in OP. Apart from that, having such a list would be great but in such case, it would be very hard to reach an user when something is important.
In SMF, the feature isn’t available by default I guess. Therefore, customization would be the only way but I guess it’s too much time consuming in exchange of the level of the benefit we will have.

[The Cryptovator]

I won't agree with OP. I know the problems, most likely users sending you PMs that you have left negative feedback. I quite agree with actmyname, besides, you may block users when you are leaving negative feedback to their profile. Those aren't listed in your trust list doesn't mean they don't need you or you don't need them. And I don't want to mix up trust network and unsolicited PM together and I believe admin will not like it as well. I believe you have to find some alternative as well mentioned above rather than expect help from admin.

[JeromeTash]

Sometimes a PM from the person you least expect to be of value could be quite helpful. Maybe you could have made a damaging  mistake while leaving a feedback, replying a thread, or you were mentioned in some post and the person who's not whitelisted wanted to alert you ASAP, but they are not able to.

I think ignoring or reporting unsolicited PMs should be OK.

[FatFork]

I agree with the majority here. That would be too restrictive and I'm not sure how useful it would be for the community.

As some have already said, I don't see any problem simply delete/report unwanted messages. You don't have that many of them, do you?