When troubleshooting email delivery, it is essential to know what the SPF (Sender Policy Framework) and the DMARC (Domain-based Message Authentication, Reporting & Conformance) records are for the domain.
You can lookup records from the Command Line:
Dig and NSLookup are two common command line tools to lookup DNS records.
To lookup DMARC
dig txt _dmarc.[EXAMPLE.COM]
nslookup -type=txt _dmarc.[EXAMPLE.COM}
To Lookup SPF records.
nslookup -type=txt [EXAMPLE.COM]
dig txt [EXAMPLE.COM]
You can also use the MXtoolbox website to lookup and test email delivery.