You don't need an MX record to receive mail: https://tools.ietf.org/html/rfc5321#section-5.1 https://tools.ietf.org/html/rfc5321#section-5.1 > If an empty lis

You don't need an MX record to receive mail: https://tools.ietf.org/html/rfc5321#section-5.1 https://tools.ietf.org/html/rfc5321#section-5.1 > If an empty list of MXs is returned, the address is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host. That said, it doesn't look like there's an SMTP server running for bar.com.