I have a few concerns about this idea.
For starters, if this were to be implemented, the whole system should be set to a toggle- as stated prior- between <On|Club+friends|Club|Friends|Off>
These settings are fairly self explanatory- depending on what it says, you will be able to control who sends you mail- if anyone.
Furthermore, the ability to block specific people would be a good idea- wouldn't it be better however if it were combined with the functionality of /ignore?
With all that said, timestamps should also be placed on every piece of mail, along with a reference time, incase server time is different from the user's.
If all of this is implemented, my concerns for the most part are eased.
My main concerns are:
That some mailboxes will just get outright flooded, the timestamp will allow users to report spam effectively. Because sending an update message every few days while a user is gone for a week shouldn't be reportable as spam- but could end up being three or so messages in a row to users who have no one else mailing them, and no timestamp to work off of. Having your inbox filled with the same message a few hundred times (exaggerated), or having a lot of nonsense messages in a short amount of time should count.
That the mailboxes of people of interest (Owner, admins, devs, staff, etc) would get flooded- but the toggle settings listed would more than fix the issue.
And well, Adding the block/ignore feature would help users in general.
The idea itself isn't that bad, but there is a lot of scenarios where it could be used bad. Hopefully these additional suggestions will make it so it can be a better user experience for all.