Customers cannot receive e-mails from Desk

Customers cannot receive e-mails from Desk

Hello,

We have this issue with one of our support tickets. When trying to e-mail a customer, they do not receive our e-mails. When looking into it, I found this error in our support@ e-mail inbox:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  customers@e-mailaddress.com
    message has lines too long for transport
    (ultimately generated from support@ourdomain.com)
    message has lines too long for transport


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How can I resolve this, please?

Many thanks