When talking about this, it's important to distinguish between the DNS reporting that there is no MX record for a domain and reporting that there is a transient DNS error and thus it can't tell if there is an MX record for a domain.
If there is no MX record, smtp should use an A or A6 record, if it exists. If there is a DNS error looking up the MX, smtp should stop and not use any A or A6 record, since that may be the wrong destination, and you can't tell until the transient DNS error passes.
