If the content is not sensitive, can I ask you to send me one of these messages? They occasionally cause nuisance to people but we rarely get any sense of what's happening. Please forward it as an attachment, so it arrives here unchanged.

xenos @ gmx . co . uk

I have seen messages that have been specifically wrapped to a certain line length by the sender's client. I hear that some mail clients use non-standard space characters so Thunderbird can't see the gaps between words and so doesn't know where to fold a line.

Neither of these sounds likely in your case given that another Thunderbird user is seeing the same messages properly.

Anyhow, here are some settings to look at. Go to Tools|Options|Advanced|General→Config Editor. If it challenges you, say you'll be careful,

In the config editor window type wrap_long into the search box. I'd expect at least two terms to appear, probably three. Ignoring the one that mentions view_source, are any of the others set to false? If so, set them to true by double-clicking.

Close the editor, close Thunderbird and re-start it. Any better?

The OP has shared with me a message demonstrating the long line effect. In this particular case, the offending text has been wrapped in a <pre>…</pre> tag pair, so quite correctly, (but inconveniently!) renders as one long line.

The mystery now is: a) who or what put the tags there, b) why was this done, and c) why do two different computers handle it differently?

I have seen this before, where press releases issued by an iPhone user had this same brain-dead <pre> tag formatting.