Some tools I personally use
An SPF record (Sender Policy Framework) is a txt record you add to your DNS to restrict where emails using your domain can be sent from. It helps prevent spammers from sending fake emails that look like they came from your business.
SPF records are an advanced level topic, so I won't explain further here.
This post is aimed at people trying to troubleshoot and create an SPF record.
Getting things right can be complex when you have multiple email servers and emails being forwarded. These are some tools I used recently that might help others (and my future self)
With the above you should be able to find what you need to create and test a complex email setup.