This looks like it could be related to an extension modifying the message just before it's sent. Can you share which extensions are in use?

Can you also check in your Outbox if the outgoing message has the same problem?

Fabián Rodríguez said

This looks like it could be related to an extension modifying the message just before it's sent. Can you share which extensions are in use? Can you also check in your Outbox if the outgoing message has the same problem?

To answer your second question first, as I previously stated, the sent message (in the Sent box - there are never any messages in the Unified Outbox) is occasionally missing some space characters, which is also the way the email message is received by the person I sent it to. I know this because if that person replies to my email message, I can see that those same space characters are missing in the quoted response.

As to extensions in use, there are three I use, due to deficiencies in Thunderbird's native features. While anything is possible, I cannot imagine that multiple random missing space characters are caused by them. The extensions are:

Edit email subject 2.1.0 Manually sort folders 1.1.1 XNote++ 2.2.13

Thanks for your help.

How have you gone about setting Thunderbird to send plain text messages?

My preference would be to set it in the account settings, but it is also possible to use the html composing mode and then downgrade the message to plain text on sending, possibly as a result of an addressee having been set to "prefers plain text" in your address book, or the Send Options is set to switch to plain text for certain domains.

Do you use format=flowed? There are some complex rules for dealing with spaces in plain text messages, particularly when they occur at the end of a line when it wraps.

TBH, I don't know which of composing in genuine plain text or relying on a downgrade is more likely to be successful.

BTW, Thunderbird stores sent messages in its Sent folders (unless you set it up otherwise.) Outbox is for unsent messages, perhaps sent using the Send Later option, or deferred messages if you're offline and which are held there until you get online.

Zenos said

How have you gone about setting Thunderbird to send plain text messages? My preference would be to set it in the account settings, but it is also possible to use the html composing mode and then downgrade the message to plain text on sending, possibly as a result of an addressee having been set to "prefers plain text" in your address book, or the Send Options is set to switch to plain text for certain domains. Do you use format=flowed? There are some complex rules for dealing with spaces in plain text messages, particularly when they occur at the end of a line when it wraps. TBH, I don't know which of composing in genuine plain text or relying on a downgrade is more likely to be successful. BTW, Thunderbird stores sent messages in its Sent folders (unless you set it up otherwise.) Outbox is for unsent messages, perhaps sent using the Send Later option, or deferred messages if you're offline and which are held there until you get online.

In the Account Settings, for the From email address, Composition & Addressing, the check box is checked for Compose messages in HTML format, and under Global Composing Preferences, General tab, under HTML, Font is set to Fixed Width, and under Configure text format behavior, Send Options, I have checked Send messages as plain text if possible, and Send the message as both plain text and HTML. No domains are specified in those settings.

Under Options, Advanced, Config Editor, Format-flowed (mailnews.send_plaintext_flowed) is set to true.

Sometimes the dropped spaces are dropped at the end of a line, or at the end of a sentence between the period and the following sentence (if I put two spaces, only one may remain after the message is sent), or sometimes between words that are not near the end of a line. I haven't determined any clear pattern as to what criteria Thunderbird is using to throw away particular space characters.

My goal is to generally send plain text messages, but that is too limiting for being able to include URL links and other items at times.

One thing Thunderbird does (not a good thing) that no other email client does that I am aware of, is to reformat a message when it is sent, in order to convert URLs from plain text to a clickable link. It may be that process, even when there are no URLs, which is causing the dropping of some space characters.

I saved a message as a draft, reviewed it, which was good, then sent it, and some space characters were dropped, so that is not a work around.

I have used other email clients over many years, and dropping space characters when messages are sent is unique to Thunderbird. Eudora had its own quirks at times, but never exhibited this behavior.

Another anomaly I'll mention, just in case it is related, is that despite my setting the preferred font to Body Text, Fixed Width, sometimes when composing a message, and pressing Return twice to begin a new paragraph, the font setting in the drop-down box will change to Variable Width on its own, which is also frustrating.

Thanks for your help.