Reply To: 5.3.0 – Other mail system problem 550-'Line too long'

Here’s a text I found very helpful for that error message:
It can be a setting in firewall or MS Exchange to attempt to defeat spam. It is related to the number of characters in a single line of the message headers. In the past, spammers might have sent email to hundreds of addresses in the To or Cc line of the email, and this restriction was created to filter out such email.

There are also limits in message body. Postfix defaults to 990 length limit, but it does not bounce email based on this. It will insert newlines to prevent the line from being longer.

So can it be that the email is just to long, or maybe its all in one line (look at the html of the template)? I dont think it’s related to easyreservations. Try to deactivate antispam bee as well.
Regards