"Invitation" emails and "Password Reset" emails not delivered

Hey Team

Our JIRA instance is: https://visualjazz.jira.com/

I'm in charge of our (~50) Service Desks and am having some trouble with emails.

The ticket notification emails are coming through OK, and the replies are logging with the system fine.

The issue lies with the "Invitation" emails, when you invite a customer to join a Service Desk, and with the "Reset Password" emails, from the Service Desk Portal.

Many of our clients are reporting that these are just not being received. I can think of at least 10 reports this year.

Is this a common issue? Is there any way to resolve this without trying to have the domain whitelisted?

Thanks in advance for your help!
Donna