your friendly neighborhood deliverability guide since 2001
Mailauth: Check SPF, DKIM, DMARC, ARC, BIMI and MTA-STS
Email developer Andris Reinman has created a cool email authentication library for Node.js that'll help you check a whole bunch of stuff relating to different email authentication bits. And it allows you to test things that might not have been so easy to test previously (like ARC and BIMI). It's called Mailauth and you can find more information and downloadable code here.