Sending email fails with "too many recipient in last hour"

[#1]

We have one email a/c failing to send with Outlook error message :

Server error: '452 <email@xyz.plus.com> recipient rejected - too many recipient in last hour'

Only a couple of emails have been send in the last hour.

[#2]

Only a couple of emails have been send in the last hour.

Did they have multiple recipients ? and are you 'off-network' i.e not on a plusnet connection ?

[#3]

Yes & Yes. 

Which is exactly how we always use this email a/c every day of the week & normally there are zero problem.

[#4]

There are quite severe limits for sending mail 'off-network'.

If you send a couple of multi recipient emails in quick succession you may well hit the restriction. It will clear after a while...

[#5]

A details of these restrictions would be very helpful.

[#6]

I'm afraid the exact details are not published

Like most ISP's, PlusNet take steps to avoid their email servers being blacklisted as senders of spam. Whilst the exact method and algorithm is not published (for obvious reasons), it is known that the sending IP reputation and the rate at which messages are being sent are major factors.
In terms of the rate, you would expect that the vast majority of messages being sent via the PlusNet servers are from Plusnet allocated IP's, with small amounts being sent from other ISP networks such as mobile networks and non UK networks. Therefore a lower rate for potential spam detection is used for non-Plusnet IP's.
Now the situation can be complicated by the fact that IP addresses are reallocated by people disconnecting and reconnecting so the IP you are allocated when connecting to a network may have previously been used to send lots of email and thus acquired a poor reputation. Additionally some networks, particularly mobile ones, 'share' public IP addresses (using CGNAT) across lots of users.
So in your case, it may not be the messages that you are sending that trigger the limit, but other users (and previous ones) on the same network that are or have been allocated the same IP that are causing the problem.

I have had experience of this myself. In the winter I used to spend a couple of months in Portugal, initially the property we rented used a mobile network for broaband and I often got the 452 error when sending email and had to wait and try again. In the later years the property changed to using a cable network for better internet, and the IP address allocated was more stable. This reduced the frequency of being unable to send mail, although it still happened occasionally.

So as you can see, the mechanism to prevent spam is not perfect. It may well be that PlusNet have changed the algorithm but if so they will, for obvious reasons, not give details.

[#7]

We have been using the same ISP via the same Public IP for over 5 years.

CGNAT is not involved.    I hope it has cleared by the morning.

[#8]

In that case, it's just the rate of sending. It should reset by the morning.