Send an email function not always delivering
What is the mail server you are using to push them out?

If it's a local mailserver (eg. postfix) you can check the log and see if it's actually really sent.

If you are using an external provider, they usually have some kind of log as well.

In any case, have you set up SPF and DKIM for your domain? if not, you are increasing the risks of having some providers that will drop your emails without any notice