Which of the following DNS record types will be used for mail transfer if the MX record is not found?

RFC 5321 sec. 5 now clearly states that:

1) SMTP clients must look up for an MX record;
2) if no MX record for domain is present, look up for an A Resource Record (RR), and if such record is present, treat it as an MX record;
3) if an MX record is present, clients MUST NOT use an A RR.

https://portal.smartertools.com/community/a313/smarter-mail-use-a-record-if-no-mx-record-is-found.aspx