Intermittent failure at start of registration process wrt email sending

Using NC v20.04 with Registration 0.70 so up to date.
Doing self-registration to generate username and password.

Every few weeks we get some problems reported by those trying to register, getting an error message saying “There was a problem sending e-mail. Please contact your administrator” or similar in other languages.

When the support team look at the problem we are never able to reproduce the failure. And our Sysadmin has not found anything in log files.

Any clue as to why this may be happening? Is there some additional logging we could turn on to throw some light?