Close ticket failed

[Johnny_LAGACHE]

When we want to close ticket (select new state "Closed" in agent reply), closing fail and some system notes appear :
==> An error occured when trying to mail the previous note, No recipient(s) were found.

This error appear too sometimes after agent notes. And a next agent note don't generate error system notes.

Moreover, "Close Ticket" button at the top works in the same tickets

What can we check, because I have already check that recipients have a correct email?

Lansweeper version : 7.1.110.6

[Johnny_LAGACHE]

My network admin check flow.
I said to me that Lansweeper use two commands to send mail : DATA command and BDat command.

Our firewall control networks flow and filter them through IPS.
DATA commands (most common) pass and BDat commands block.

Can we configure something in LAnsweeper to always use DATA command?

Best Regards