SORBS blocks on outgoing email

[#1]

When will PlusNet address the blocks on some of their mail servers. 
Today I had 6 emails returned with the dreaded "550 relay not permitted for your IP" error. 
This happens sporadically but is a real pain as sometimes you don't see the rejection until the next day, or worse if you are going away from home.
Sometimes the issue resolves itself on a resend - at other times it needs the user to disconnect and reconnect to get a different IP address.

[#2]

Have you checked whether the Plusnet mail relays *were* on the SORBS blacklist? Often the cause of this problem is that the IP originating the email (your PC) has been checked rather than the mail server. What IP was reported with the "550 relay not permitted" error message?
The present status of the Plusnet relays appears OK to me.

[#3]

I don't know what the IP's of the Plusnet outgoing mail servers are.  I am using plain vanilla Plusnet  - ie I don't have my own mail server so I am not sure how my own ten current IP address comes into it.  I have raised a ticket on Plusnet's support site on a number of occasions and once th eresponse was that they are aware that some of their mailservers are on the SORBS blacklist - hence the issue.  Also, sometimes the problem will go away without me getting a different IP address on my connection - which it would not if the issue was my own IP address.
Interestingly the problem has not hit my son in Leicestershire but it seems to hit me in Sutton, London about every three or four weeks.  That means that my support ticket has been automatically closed before I can update it with another event.

[#4]

PlusNet's e-mail servers have been blacklisted by SORBS several times in the last few weeks.
Apparently, work is being done to stop this happening in the future.