Outbound emails intermittently bounce back displaying the "recipient domain must exist" error for the same domain.
This is due to missing DNS Records for one of the MX, preventing outbound delivery of email. For example;
mirapoint>dns lookup example.com type=mx
MX 100 server100.example.com.
MX 200 server200.example.com.
MX 300 server300.example.com.
mirapoint>dns lookup server100.example.com type=a
mirapoint>dns lookup server300.example.com type=a
NO No data
If one or more of the.MX records is unresolvable, Mirapoint does not always relay the email.
The workaround is to modify the DNS and add an A record for server300.example.com.
If there is no control over the DNS in place, contact the 3rd party domain owner and ask them to correct their MX records.