I believe sending a PM causes one copy of the message to be put in your Outbox and another copy in the recipient's Inbox. Deleting the copy in your Outbox therefore would not prevent delivery.
Any two members could test this theory by having one send a message and then immediately delete it, before the other member tries to read it.
Not by yourself via the forum software. arn (the site owner) or somebody else with suitable administration privileges could in theory expunge messages from the underlying databases, or erase PMs for a given member with an administrative tool, but we have never done such a thing before, as far as I know.
We moderators never read or access PMs between members, other than those sent to us.
If you are asking because of a particular message you sent, and you consider it an emergency, send me more information by PM, even though it may be too late.
Yeah, that was kinda a sweet feature. You could check the status of messages sent to other AOL subscribers and delete them if they were still 'unread.' It was definitely nice. Not just being able to delete the messages, but also see if someone hasn't responded because they haven't read something as opposed to them just being lazy or mad 😛
There are a few PM's I've sent, these forums and elsewhere, that in hindsight could've benefited from such a feature and/or a little 'cooling off' period of self moderation/common sense😱