This document explains the problem: 
https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/non-delivery-reports-in-exchange-online/fix-error-code-550-5-6-11-in-exchange-online

I will omit my thought on remote admins. 
A very simple rule in communication is: be strict when sending, be tolerant 
when receiving.

Regards,
Tonino


Il 28 agosto 2022 19:24:13 CEST, Jeff Koch <jeffk...@intersessions.com> ha 
scritto:
>Further information:
>
>The sender also got the following message from his exchange mailserver when 
>trying to send to our mailserver:
>
>Your message contains invalid characters (bare line feed characters) which the 
>email servers at ......com don't support
>
>Jeff
>
>
>On 8/28/2022 12:45 PM, Jeff Koch wrote:
>> 
>> Never heard of this before but an email sender to our mailserver got the 
>> following error message:
>> 
>> 550 5.6.11 SMTPSEND.BareLinefeedsAreIllegal; message contains bare 
>> linefeeds, which cannot be sent via DATA and receiving system does not 
>> support BDAT
>> 
>> Does anyone know whether the qmail toaster supports the SMTP protocol BDAT 
>> command ?
>> 
>> Thanks,
>> 
>> Jeff

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