email – Mail Bounce due to Invalid string MX record

I’m new to managing a mail server and I encounter an issue with sending a message to a certain domain.

Our emails are bouncing back on us with the error of the Invalid MX record string.

I’m using Lotus Domino.

Below the message I got on bounce email:

[<00>] XMail bounce: Rcpt=[];Error=[Invalid MX records string format]

[<01>] Error sending message [1620000931566.5a6ffb70.1089.1bb0f.protect] from [].

ID: <21050300-6320-0000-0000-000002A2C8E0>

Mail From:

Rcpt To:

Server: []

[<02>] The reason of the delivery failure was:

Invalid MX records string format