There has been some discussion of relay boxes. This page provides an example of a relay box. So in my reading of man_made=street_cabinet wiki page, it would appear the suggested approach would be man_made=street_cabinet + street_cabinet=postal_service. I added an example of such a relay box here.
I also created a street_cabinet=postal_service wiki page
Thanks, I was unaware of street_cabinet=postal_service, which seems to be more common than amenity=relay_box. It seems like both tags came up in the October 2014 tagging list discussion about street cabinets. However, I’m unsure about the relative advantages of either approach. It also isn’t clear to me how to distinguish between postal relay boxes and other postal storage boxes. That would be a minor point except in the kind of detailed 3D renders we were discussing previously.
https://commons.wikimedia.org/wiki/File:Storage_%26_Relay_boxes_(7855000056).jpg
If the need to distinguish , you might do eg street_cabinet=postal_service + postal_service= =relay? Anyway the format postal_service:*= was raised in Proposed features/unified postal service tagging - OpenStreetMap Wiki to re-unify post_office:parcel_*= and parcel_*= wording. Semi-relatedly, fire_service= for firefighting.
For anyone who missed it, relay box and cluster box units were discussed for Canada last month locally [Talk-ca] Community Mailboxes and Relay Boxes Tagging and internationally [Tagging] Tagging of community mailboxes (cluster maiboxes) , but mainly ended up for =post_box vs =letter_box.