Check your SPF record.
Find out if a domain's SPF record is valid, how many of the 10 allowed DNS lookups it uses, and whether it really blocks servers nobody approved.
How SPF decides
What SPF does
SPF (Sender Policy Framework) is a TXT record on your domain that lists the servers allowed to send email as you. Receivers compare the sending server with that list.
The 10-lookup limit
Every include, a, mx, ptr, exists and redirect in the record costs the receiver a DNS lookup, and includes can contain more includes. Past 10 lookups, SPF fails for every message. It is the most common SPF error, and the checker counts it for you.
How the record should end
~all (soft fail) marks mail from unlisted servers as suspicious and is the usual choice with DMARC. -all (fail) rejects it. ?all says nothing, and +all lets anyone send as you.
One record only
A domain can have only one SPF record. When you add a new email service, add its include to the existing record instead of creating a second one.