• http://kb.mozillazine.org/Plain_text_e-mail_-_Thunderbird#Flowed_format
• http://kb.mozillazine.org/Mail_content_types#Disabling_paragraph_flow

• Edit > Preferences > Advanced > General
• click on 'Config Editor
• type: flowed

What Values do you have for the following:

• mailnews.send_plaintext_flowed Value True
• mailnews.display.disable_format_flowed_support Value False

Thanks for your reply!

I had read the help articles before. These are my settings:

• mailnews.send_plaintext_flowed: true

The first one is set to true = default.

• mailnews.display.disable_format_flowed_support: true

I have set the second one to true because I want to have e-mails that I receive being displayed wrapped (in Thunderbird on my laptop). This seems not to be about sending e-mails (I had it set to false before with the same result).

Some more information: I have just tried to send an e-mail from another e-mail account (also in Thunderbird). In this e-mail, format=flowed works correctly! Are there any account specific settings regarding format=flowed?

Is it possible that my e-mail provider (that one I tested first) reformats my e-mail?

That's in the header of my web.de e-mail (not working):

• User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0
• MIME-Version: 1.0
• Content-Type: text/plain; charset=utf-8; format=flowed
• Content-Language: und
• Content-Transfer-Encoding: quoted-printable

And that's in the header of my university e-mail (working as expected):

• User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0
• MIME-Version: 1.0
• Content-Type: text/plain; charset=utf-8; format=flowed
• Content-Language: de-DE
• Content-Transfer-Encoding: 8bit

So maybe the Content-Language or the Content-Transfer-Encoding is important?

Try setting TB to wrap plain text to the window width by setting mailnews.wraplength to 0 (zero). I've found that makes messages display correctly in mobile mail apps on phones.

Thanks for your answer!

That's definitely fine for smartphones. But members of a newsgroup told me that they prefer wrapped lines (no matter if hard or soft wraps) because reading full lines on a large screen is not pleasant.

So I still prefer format=flowed because that the perfect solution for everybody (smartphone and PC users).

But not wrapping at all is still better than hard wraps. If I can't find out why the trailing spaces are missing, I'll go for this workaround!

If these newsgroup members want to read mail in narrow columns on large screens, they can resize the window or pane of a message that wraps to the window width. Better to choose a format that is preferred by most users, rather than conform to a format that suits a minority.

Regarding difference in the 'Content-Transfer-Encoding:' showing 'quoted_printable' and '8bit'

The preference 'mail.strictly_mime':

If set to 'true' it will force 'Content-Transfer-Encoding' to 'quoted-printable' or 7bit.

However, when composing in HTML but sending only as plain text, 8bit is used due to conversion. It is possible to have the account set able to compose in HTML, even if you do not use any of the formatting options and still send as plain text only. Maybe the university is set up to allow composing in html and sending in both Plain Text and HTML or only Plain Text.

Are the accounts set up to allow composing using HTML?

• Right click on mail account name in Folder Pane and select 'Settings'
• Select 'Composition & Addressing'
• checkbox for 'Compose messages in HTML format' - Is this selected or not ?

Are you only sending as 'Plain Text' only or both ?

• Edit > Preferences > Composition > 'General' tab
• click on 'Send Options'

mail.stricly_mime is false.

In both accounts the checkbox for 'Compose messages in HTML format' is not selected.

Edit > Preferences > Composition > 'General' tab > 'Send Options'

• 'Send plain text if possible' (my translation) is selected.
• If somebody can't receive HTML mails: 'Send both HTML and plain text'. (my translation)

I have sent emails using msmtp (command line) from both accounts. Again, the web.de e-mail contained only hard wraps, the university mail had soft wraps.

So maybe it's not about (Thunderbird) settings but web.de is messing up my mails on server side?