DNS MX record and SMTP server
Solution 1:
Your SPF record specifies two mechanisms, and the source IP for mail must match at least one of them:
- Either the
Arecord for the domain (in this case the apex, since you don't specify anything aftera), and/or - One of the
MXrecords must resolve to the source IP.
Assuming mail and smtp resolve to different IP addresses, removing the MX record for smtp means that it is no longer satisfies the mx mechanism.