First, we're volunteers here. If you want a formal, official response, you need to go elsewhere. However, there is an RFC in place for it (https://www.rfc-editor.org/rfc/rfc8792.pdf ), it's been a common line length for decades, many people prefer it for it's tighter construction, and there's even an addon to enforce it. So, just looking from the outside, I would guess that the option is there as a user convenience. And, even if it's a poor explanation for you, it's still there. I should also add that it's an option in other email clients, such as TheBat and Becky!, and possibly others. Just my two cents...

Thank you for your time and patience sir. Sorry I was a bit mad :< Is a decent explanation. I'm not sure what to believe, I don't know world-wide but Google mail is really popular around me and, to make emails send from Thunderbird to google mail format weird (even if Google are in the wrong) is an odd decision.

The line length is only used for plain text emails. So basically it is not used at all.

The issue is Thunderbird under the hood converts HTML mail to plain text if there is no formatting. That is the problem and when I submitted a bug requesting a change of default the bug was summarily closed. Apparently the folk in the development team are old UNIX users and they like plain text.

Please explain to me like I am 5, why is this ON by default?

Many software developers keep in touch with other people on their project development team by being "subscribed" to lists of group email called "mailing lists". Software developers have used mailing lists for decades, and many read their email through simple, minimal "command line" programs like emacs, mutt, etc as opposed to larger graphical e-mail programs like Thunderbird and Outlook.

Those big email programs tend to send mail with HTML formatting, and HTML-formatted messages (or lines that stretch beyond 72 characters) are often unreadable or unpleasant to read in text-based mailers. Here is an example from an actual mailing list showing how much cleaner the mail looks when HTML is stripped and lines are wrapped:

https://marc.info/?l=openbsd-ppc&m=171686293612874&w=2

and what someone who reads mail in a program like mutt might see:

https://i.ibb.co/L1vgW9T/0-v-Y8-Tcp0a-Gy-DKJc31.png

What it looks like when it's not wrapped:

https://i.ibb.co/FgngQpF/mutt.gif

This mailnews.wraplength "feature" looks a lot like is there to signal to other non-Thunderbird users to use Thunderbird. I really really hope there is a good explanation for why `mailnews.wraplength` exists and is on by default.

It may be. The idea here might be to attract devs with "You can continue to contribute to software development mailing lists AND, when the situation doesn't require plain text, also enjoy all the other features Thunderbird has to offer." Just a guess.

I hope this helps.

Matt seems to have mostly addressed the heart of the issue here already though. Old UNIX greybeards do love their plain text.

Ok, I understand, but,

OBJECTION! Whether rightly or wrongly, because "those big email programs tend to send mail with HTML formatting," it means that the vast majority of people use email with HTML formatting. Because of this, it's up to the older email clients to ensure that HTML formatting displays correctly.

Moreover, since Thunderbird is one of those "big email programs," it should have HTML formatting enabled by default, with an option to turn it off if preferred. This is out of my belly, I don't know, but I assume something like 95% of emails are sent with HTML formatting. How could Thunderbird have this off by default and such an obscure way of turning it off?

Ok, I understand, but, OBJECTION!

Time to backup. Lots of discussion and none of it is helping your understanding. No one really cares about the dinosaurs using Mutt or EMACs. If Thunderbird developers are not eating their own dog food and using Thunderbird for the communication needs, I have no interest in their opinions. Neither should anyone else.

1. Change the Thunderbird default. Go to the setting and search for HTML. Change the setting from automatic to HTML and Plain text. That allows the email to be read in whatever type of mail client the recipient has.

I do not consider that either obscure or difficult. What is difficult is a world community that as a whole does not even know there is such a thing as plain text messaging to know there might be a setting to look for.

2. Forget the line length as it is only relevant to those using plain text. If you change your sending format to text and HTML, no one will see the plain text unless their mail reader is not capable of HTML in that case they will be pleased to have their mail wrapped at 72 characters.

BTW Just to set an historical context. IBM 3270 mainframe terminals operated in 80 columns. Given that "some" of those columns were used for control codes and basic indents or frames wrapping at 72 columns was the "industry standard" See https://en.wikipedia.org/wiki/IBM_3270

I last used a 3270 emulator about 20 years ago, but I know folk that are still using 3270 emulators to access mainframe applications in the corporate world. So the technology may date to 1971, but it is still out there in niche environments, often using other technologies from the same period like COBOL (Government and in the financial world particularly)