Although I appear to be receiving my emails correctly and on time, I have been told by several contacts that when they send me an email, they get a message saying it hasn't bee?n delivered, it's something to do with my server?
Without seeing the message that your senders get it is hard to say what is happening. It could be one of several things. Most likely it is simply a message about a delivery delay because VM are rate limiting connections from the sending servers. But that is only a guess. If you can get a copy of one of these messages post it in this thread (with all personal identifying details removed) and we'll help further.
Superuser 2017/18 Use Kudos to say thanks Mark answer as "helpful" only when the problem is solved Please don't send me private messages unless I ask you to. I do not work for VM. The advice I give is based on my best understanding of VM policy and practice. You rely on it at your own risk.