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?)