You mention that the message is sent in the copied format. Please clarify. Details will help. From what you wrote, my inference is that the message was sent in your preferred email font. Since the paste was without formatting, it used the thunderbird setting. If you were pasting from a different font, and wanted it preserved, I would expect to paste formatted. Thank you,

Thanks for your Question! Normally I write my messages in Arial. When I Copy from another source and paste it like I said, TB is showing me the Text in Arial. Exactly what I want cause I pasted unformated Text. When I do send this mail and look in my sent messages I see that my text is still in Arial but the copied part switched to e.g. Times New Roman but it should be Arial as well.

I am not understanding. You state that the text is in Arial in the sent message, but you also state the copied part is in times roman. but the text that you copied was in Arial and you stated it remained in Arial. So what is in times roman? Can you create a sample screenshot? Is times roman your default font?

My normal Text is Arial. No matter from where i copy a text, i want it in my mail as Arial. When i do copy e.g. Times New Roman, i paste it without format in my mail and it is shown correctly as Arial. So my whole Mail in edit mode has Arial as font. Next step i send my mail and looking in me sent message folder. When i open the mail everything is in Arial except my copied part... that is written in Times New Roman or another font.

I cannot duplicate this. pasting without formatting removes font settings, so they cannot reappear after being removed. You can investigate this by viewing source of the message to see what HTML was added or changed. To see it will yo're composing it, I suggest installing the ThunderHTMLedit addon from https://betterbird.eu/addons/ That will let you see the HTML prior to sending the message and you can compare that with the HTML after sent by highlighting message and pressing cntl and 'u'