When you have multiple accounts on the same server, it's important to verify that there are as many smtp servers defined as the number of accounts, and each account is set to send on an smtp with a matching User Name. Have you confirmed this?

https://support.mozilla.org/en-US/questions/1350593

On a second, possibly related point, you shouldn't be seeing certificate errors if you follow the recommended practice of excluding the TB profile folder from AV scanning, and disable scanning of secure (SSL) connections.

Plus, confirm your Yahoo settings:

imap.mail.yahoo.com, 993, SSL/TLS, OAuth2, email address; smtp.mail.yahoo.com, 465, SSL/TLS, OAuth2, email address.

each account is set to send on an smtp with a matching User Name. Have you confirmed this?

Yes, each account has a corresponding smtp with it's email address, Yahoo, Gmail, Outlook.

On a second, possibly related point, you shouldn't be seeing certificate errors if you follow the recommended practice of excluding the TB profile folder from AV scanning, and disable scanning of secure (SSL) connections.

The certificate only happens with Yahoo on account setup. 4 popups for exceptions, all same. Only AV is Windows Defender, and have dissabled in the past and made no difference as I also tried in Safe mode and same.

These accounts have existed for years but I do not send many emails with attchments so I don't know when it started to fail. I can Receive text and attachment Yahoo emails and I can send text Yahoo emails, but I cannot send Yahoo emails with attachments if there is more than one account on TB.

I use Windows Defender and have never done any changes so I dn't know about scanning SSl connections.

Plus, confirm your Yahoo settings: imap.mail.yahoo.com, 993, SSL/TLS, OAuth2, email address; smtp.mail.yahoo.com, 465, SSL/TLS, OAuth2, email address.

Exactly as yours only POP and as created by TB on account creation.

I have a test PC with a clean w11 that I installed TB and setup two Yahoo accounts and I can send attachments. I then exported that profile and imported onto my main PC and it failed. I cannot send emails with attachments.

I have been at this for two days.

This is killing me. I just uninstalled TB and appdata TB folders and reinstalled. Created just one Yahoo account and now it can't send a Yahoo email with attachment as it could half an hour earlier, actually this was a reply to an email that had an attachment, but why? I'm done!

Why just Yahoo and only if sending an attachment and not my other accounts, Outlook, Gmail. Just Yahoo?

I can't give you a definitive answer, but why does the exception dialog refer to yahoo.com.au, if the servers are all *.mail.yahoo.com? https://au.yahoo.com/ is Yahoo Australia, and I would guess that email accounts are @au.yahoo.com.

I would still add the TB profile folder as an exception in Windows Defender/Security.

I can't give you a definitive answer, but why does the exception dialog refer to yahoo.com.au, if the servers are all *.mail.yahoo.com? https://au.yahoo.com/ is Yahoo Australia, and I would guess that email accounts are @au.yahoo.com.

I have no idea all I can say is that that's how TB creates the accounts and I can attest that is how it's been since about 2015 when I first started using TB. Whilst this may be a bug it wouldn't be the problem now for those Cert started appearing now during fault finding.

I would still add the TB profile folder as an exception in Windows Defender/Security.

Why now, it's been working fine for over 8 years.

Some success: Ok, some success. This would have started around June-2024. I don't send many attachments so I only now have found the issue. The only thing that changed around then was that I changed Yahoo in Thunderbird from 'Normal' One time third party App Password to OAuth2.

So after all the above and days of failure with OAuth2 I reinstalled TB and regenerated Yahoo's one time passwords. I can now send attachments.

It's SMTP and OAuth2 with Yahoo that's the problem and I seem to be the only one with this problem. Only thing I changed was OAuth2 > Normal Password > New one time password for both POP and SMTP.

If you have OAuth2 authentication, enter the account (not app or one-time) password in the OAuth dialog. To reset the authentication, remove the entries for the account from Saved Passwords in Settings, restart TB, complete the OAuth process to allow access.

If you have OAuth2 authentication, enter the account (not app or one-time) password in the OAuth dialog. To reset the authentication, remove the entries for the account from Saved Passwords in Settings, restart TB, complete the OAuth process to allow access.

Sorry dude but you missed the the whole issue. If I was using the wrong password I would not have been able to send or receive anything yet alone send and receive text emails without attachments.

Again. I can not send a Yahoo email with an attachment as now discovered if using OAuth2 authentification.

Sorry but as you have already determined, you appear to be about the only person with the issue.

I have multiple accounts with yahoo using .au domain names. But my outgoing server is configured as smtp.mail.yahoo.com port 465 using oauth authentication. I do not get any sort of prompt for a certificate exception. Perhaps check the issuer in the view button