My messages are not being received reliably and since a couple of days I keep getting mail failure messages.?

Since a couple of days I can't rely on my messages being received. I keep getting mail failure messages from various recipients, but not all. Some then turn out to have received my message after all, some never got it. Some recipients get some of my emails, but not all. I've also had at least one recipient tell me they in turn had difficulty sending me a message. How do I solve this as I now can't rely on messages being received and have to keep ringing people to make sure they got it which is defeating the purpose of email.