What version of Thunderbird are you using? Did you update Thunderbird today or last night before closing Thunderbird?

Started this morning after new "updated" 91.12.0

Worked fine yesterday with older version. Only effects sending mail. No problem with receiving or IMAP.

Do you use 'send receipt' automatically when requested or did you use it on the occasion that prompted that error?

What OS are you using ?

Menu icon > Preferences > Composition click on 'Send Options' What is currently selected ? Are you sending using only Plain Text or only HTML or sending as both ?

Can you do some testing? See if only sending in plain text causes it? Or whether only sending in HTML causes it ?

Do you use 'send receipt' -no What OS- Windows 11 Pro last update 18 June Build 22000.795

click on 'Send Options' What is currently selected ? Are you sending using only Plain Text or only HTML or sending as both ? HTML only Standard Global Composing Preferences "unchecked" HTML and tried to send- same error so not HTML cause

I know I asked you if you had used Return Receipt, but check to see if this set up automatically Menu icon > Preferences > General click on 'Return Receipts' Is this checkbox selected : 'when sending messages, always request a return receipt'

When you send an email in the 'From' field it says: display name <email address> Do you have a display name which uses an accent over a letter eg: Equalité ?

re : Subject Can you double check the Subject line on failed message - does it have a hidden space at the end ?

Do you use a signature? If yes, Is that signature from a html file which you created using Thunderbird Write window or something created in eg: MS Word?

It does seem very odd that qmail suddenly does not like something. It's a question of trying to work out what is triggering it.

If you use Notepad++ Select the email you sent Use Ctrl+U to see the source in a new window. Right click and 'select all' then rigth click and select 'Copy' Open Notepad++ and paste in contents. See info to force it to show the end of line characters :

• http://sql313.com/index.php/43-main-blogs/maincat-dba/62-using-notepad-to-change-end-of-line-characters

Look to see where you see the error qmail pobox mentions. End of line not ending with CR LF

Post image showing the error so we can see where it is occuring.

Create an image

Return Receipt - box NOT checked

Signature: yes- I have it typed in - not copied- no logo or symbols

Agree odd that after this update only and not before

Do not use Notepad

Error Message pops up: Sending of the message failed. An error occurred while sending mail. The mail server responded: See http://pobox.com/~djb/docs/smtplf.html.. Please check the message and try again.

Only lets me click ok which does nothing except get rid of pop up. Same error when sending again.

SOLVED!

Although Network Solutions says use ports 587 & 2525 this update caused an issue with 587. If port is changed to 2525 all emails go out without issue.

I do not understand the techie stuff on ports but will not argue with success.

Thank you Toad-Hall for working on this.

You mention port 587, but nor sure what other settings you were using. It is usual when sending using 'Port 587', you would use 'Connection Security: STARTTLS' and 'Authentication Method: Normal Password'.

Connection security: none Authentication: Encrypted password

Network Solutions has instructions on their web site. Can't believe everything you read.