I have been getting this same issue for the last few weeks. I send a newsletter to a contact list and have been doing so for the past year until this error started to appear. I'm using a home network and have 2 modems to choose from: BT and EE. I am getting the same error on both even after turning off to obtain a new IP address.
This is driving me mad and affecting my business plans. All suggestions welcome.
My newsletter is text, a screenshot and a PDF attachment. I've tried removing some elements but still get the error. And today I just tried sending just a weblink to my LinkedIn page and that came back the same..
