Emails says sent but not appearing on WP Mail logging

I cannot reproduce this problem but it has been reported by three clients so far. The contact page says the email has been sent but there is no corresponding entry listed in WP Mail Logging. I had the client test and log time and date over several hours. On two (of twenty) occasions the test was not received nor was there a log entry.

My own tests never reproduce this error.

The clients discovered this by being told by their cuatomers that they (the customers) had sent an email and received no reply. On a very few of these there was no corresponding log entry. My assumption was user error but in each case they report haing received a message sent confirmation.

Each client uses a different mail service – one Easy SMTP, another Mailgun, and another Mailpoet.

My Host says look at the mail service since they hold the logs, the mail services say they received nothing.

This has made one client ask me to remove the contact page.

Any advice or ideas on this?