Emails in sent box appear to have been sent, but are never received

I have an intermittent problem which is causing me no end of trouble for my business. Since about November last year, it seems that a certain percentage of my iCloud emails are never delivered to the recipient. There is nothing to identify failed messages -- they appear in my Sent box normally and as far as I can tell have been delivered normally. Some of them have been "new" messages, starting a new thread (which I suppose may possibly be trapped by someone's spam filter) but the majority are replies to another email, which I think are very unlikely to have been filtered as spam. I believe the people telling me that my emails have simply never arrived in their inbox at all. Messages with attachments and without attachments both fail. I am using the standard Apple mail client on my iMac.

This only happens with my Apple email account -- although to be fair I send much more email through that account so it is not necessarily a fair comparison. It is nothing to do with Apple filters for obscenity -- there is nothing at all objectionable in the emails that appear to be failing. Most of them are very standard business communications.

Has anyone found a solution to this problem? I wonder if I need to BCC myself on every message to see that it has come through (and even then, would that prove that all recipients received the message?)

Many thanks to anyone who can help