One of my regular mailings and an email contact group (lots of people with same email address ending) were rejected as spam by the Plusnet Spam filter starting soon after 18th September 2015. I had the lowest level of spam filter on and made no changes prior to this. Plusnet during on-line chats said that the emails were not blacklisted and it was my firewall. I don't have one set up! They said they would white list the address for me, but one won't load into the system probably due to spelling of the address.
Here is a tip that seems to work as a last resort!
I have not white listed anything.
I decided to set up a Junk file, switch off the spam filter in Webmail and accept tagged spam. The regular mailing and one of my contacts turned up in the junk file yesterday tagged as spam.
This suggests that the Plusnet spam filter is rejecting some good messages as spam and it can be difficult to assign them as good messages with the tools provided by Plusnet. The only option seems to be to accept them in a mixed basket of all other spam
The next stage for me is to ask Plusnet how to turn the "good" spam messages into regular "good" messages. At least I have got them through and can prove they were being rejected as spam.