Not receiving email notifications from WHM

Running a new server on AWS Lightsail for the last 2 weeks, and I haven't received any email notifications from WHM yet. Tried changing the email address, double-checked to make sure email notifications are turned on, but can't figure out what's wrong. I run 10+ WHM servers, this is the first time I'm having this issue. It's also the first time I'm using Ubuntu, all my other servers run on CentOS. Not sure if this has anything to do with it. Can I get some help to resolve this?