Hi this is a classic problem which raises when the provider changes his mail filters. Are you using the SMTP? The test from the SMTP panel returns “ok” but the email is not delivered? Tests from the status panel, report any errors? If not then the email is moved to the mail server but the mail server is dropping it. Try to ask the provider.
If you want to make a test, signup for a free account on mailtrap.io. Use their SMTP parameters and do some tests. Surely you’ll find the email on their console (the grab alll emails and show them on their console). If the emails are there is means the plugin is delivering them to the mail server correctly.
Let me know, Stefano.