My first guess would be to wonder if there’s any common thread with the recipients.
In the example you gave, have you ever successfully sent email to that person from anywhere? The same question would apply to the other bounces.
Whenever I encounter some odd problem like that, the first troubleshooting steps are to start eliminating variables – like the sending server in this case, or finding out if the rejections have a common pattern.
Another possibility is that someone thinks you are spamming them – since you mentioned 2 or 3 out of a 100 emails. I doubt if I send 100 emails a month – let alone in a day.
Write back – this should be interesting,