IMHO, only unless the OP used an empty boundary in multipart message.

https://en.wikipedia.org/wiki/MIME#Multipart_messages

It would be easier to tell if we saw an SSCCE of the problematic email

Outlook is not the only client to ignore standards, Thunderbird is mentioned in the WP article.¹

My approach was to go for an as simple as possible "standard" which tested well with all main mail clients. (My paying clients insisted on freely composing HTML emails in TinyMCE with embedded images and send them to hundreds of recipients in a group of companies with > 300K employees in total)

Multipart in combination with base 64 worked very well. And of course a unique boundary (very similar to here-docs)

Cheers Rolf
(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery

¹) And to be fair, old RFCs are prone to ambiguities. Sometimes clients established new "standards" for extensions which were only codified later. While MS was notorious for this, it's not the only player in this game.

In reply to Re^3: Is ChatGPT like having a thousand monkeys? (Blank lines in emails) by LanX
in thread Is ChatGPT like having a thousand monkeys? by talexb

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