Do your messages have a signature, especially one with a remotely-hosted image, or do messages contain many links relative to text? Otherwise, you might have to contact the mail server admins to find out their spam policies. Recipients (including yourself) can sometimes avoid this by whitelisting senders by adding them to webmail contacts.

No signature or remotely-hosted image. Some or most of the mail that was kicked back was merely text, no images or links. I use a VPN and found the problem went away when I connected using a different gateway. I think I need to discuss this with my VPN provider and see what they say. Thanks for your reply, I appreciate it.

A VPN, for obvious reasons, masks the true location of the user, which conflicts with some mail providers' authentication checks. It adds nothing to, and often interrupts, email connections.