I've had a similar issue for the past week. e-mails from a reputable business I've worked with for years have been going into spam. When I've found the e-mails and replied, something in their footer (signature block) was being seen as dodgy and the e-mail was not sending until I deleted the footer. Next day, my e-mail address was blocked by Virgin. They thought I had malware but I do not.
I think they have upped their security and are reacting to false positives.