email "greylisted"

Maybe several reaons, depends on, where is the sender of the NDR.

If the sender is the recipient server, that you may have hit any deny rule on this server, may be the server itself or a spam or virus filter. Try to send the mail without attachement, maybe the attachement is blocked.

Greylisted may mean, that all futher mails from the same sender address will be blocked for a limited time.

Error code means "mailbox unavailable". This can be due to the reason above, but also other reasons may possible, like full mailbox or whatever. Also it maybe the the address do not exist anymore.

In addition to my post above:

Greylisting does not effect emails coming from trusted or recognised email addresses.

'Tarpitting' is the term given to delaying subsequent emails from the same sender or mailserver.



The message you received indicated that it was greylisted for a period of 5 minutes. If your mailserver retries after the 5 minutes, it will go through successfully.

I thought Mike Carroll explanation was easy to follow and succinct.  Thank you to the others as well for your efforts.