outbound message rejected due to spam or virus

[#1]

I think it is more likely to reflect greylisting. If the Plusnet relays were short of resources there would likely be a lot of posts on here.

For greylistited responses it's up to the sending MTA to try again after a suitable delay - is your system configured to do that?

[#2]

David's right. My guess is that you're hitting the recipient limit for allowed messages from your IP address. What sort of mail volumes are you smarthosting via the relays?

[#3]

Could be, I did a quick count and there are approximately 360 messages over a 24 hour period

Though a lot of them will be spam that I fully expect relay.plus.net to reject.

Maybe I should do some pre-filtering

[#4]

If you are trying to send spam messages via the relays it wouldn't surprise me if as well as these being rejected non-spam messages are subject to lengthening periods of greylisting.