Hey Sabatino, 

Thanks for reaching out to SmarterTools regarding this issue. I took a look but I didn't see a download link for 120 on their website it looks like they only have 115. Are you also able to replicate this issue on the current public release of Thunderbird?

Thank you, 

yes

I confirm that the behavior is the same with version 115.4.1 (32bit)

Hello Sabatino! Can I have you double-check what encoding options are selected for composing/reading messages in eM Client and Thunderbird in this scenario? If one or both isn't sending font data in the correct encoding this could explain the behavior you're seeing in testing across different versions. Let me know what you find! :-)

Hi Kyle.

Sorry, I understand that in English there are no words with accented letters

So I'll give you this test.

The problem occurs only when sending from thunderbid

Thunderbird defaults to Auto Send format

If you prepare a message by writing in the body

Use of €

He will send it in plain text format

And in this case the problem I described to you occurs. You can try it

If you put other elements in the body, such as a photo, he will send in text and html format

and in this case the problem does not occur.