Do the mails actually hit the mail server?
They don’t. And no errors in Umbraco Logs or umbracoLog table or anywhere.
And are you on the latest version of Umbraco and workflow?
When the problem started a couple of months back, we were. We aren’t now because of the releases from both Umbraco and Workflow. But they are minor updates so I can only assume won’t affect those issues. But, it is fair to ask to be on latest which I will be scheduling in as soon as I can.