I had the same issue a few weeks ago. My emails were being flagged as "spam". The problem - whatever it was - was resolved within a few days. I have no idea what the issue was. But I can tell you that I was sending long emails with "links" which may have been the issue BUT when I sent the same to my business email it went through with no problem. The error message did say it was an issue with the recipient's server but it seemed off that a few different folks would all be on the same server. Wait a couple days - it'll probably be resolved.