Hi.
I have developed a web page, where it is possible to book a meeting with a advisor. When you book a meeting, I generate a calendar invitation and sends it to the person who booked the meeting's email.
This works like a charm, but some times when the person then accepts the invitation, the accept response email fails to send.
The person who accepted the invitation receives an error message, and may think that the meeting wasn't booked, whicj is not the case. This is unfortunate.

The error message 1:

Can not deliver the message you sent. Will not retry.

Sender: <xxx@blabla.dk>

The following addresses had delivery problems
<pf@yyyy.dk> Unable to connect to 10.xxx.xxx.104

The error message 2:

Can not deliver the message you sent. Will not retry.

Sender: <mf@zzz.dk>

The following addresses had delivery problems
<pf@fdsfsd.dk> Unable to connect to 80.xxx.xxx.124



Any ideas?

Best regards
Peter