spf.frontbridge.com has no MX records.
Question and response
Question
dig spf.frontbridge.com. MX
Answer
The response has no answer records.
Authority
frontbridge.com. 60 SOA ns1a.o365filtering.com. isgnoc.microsoft.com. 2019100702 10800 3600 604800 60
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0