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

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

Tell me about it. I have two other PC's on this network all W11 all running same TB all tested with same Accounts/Profiles and this one PC is the only one that will not send Yahoo with attachment when using OAuth2 authentification. I even created a fresh working yahoo account using my credentials on another PC and copied it over to this one and it failed. I reinstalled W11 twice and TB countless times dissabled and enable everything I can find.

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

Exactly how I have it, exactly how TB creates them whether using OAuth2 or Yahoos 3rd party one time passwords. Only with me OAuth2 times out if I send an attachemnt with Yahoo.

The cerificates only happens when creating the Yahoo accounts. Once done it's done. You will not have seen it happen unless you start fresh. This also happend on the other two PCs when setting testing accounts for this nightmare. Also If you have an existing Yahoo account and create new ones you will not get the certificated request. It's already in your system.

As an obsolute spaced out Idea here is a theory since nothing else worked:

This PC has had TB since 2015 some time after 2015 Yahoo changed the rules and wanted one time passwords then years later it never requested but has OAuth2 available. I changed to OAuth2 because it was required by Outlook this year so I did all accounts. If I hadn't done Yahoo all would still be good.

So, From what I can tell/experienced Yahoo knows my computer ID and TB, what if there is a conflict between old and new. Yahoo expects from my PC the OTPWs since it's still in their accounts and knows my computer so when I changed to OAuth2 it kept expecting My PC to login via OTPWs but is confused when using OAuth2 with the right credentials. As a fail safe to prevent people from loosing email support as a guess it falls back to text only send and receive.

I go back to OTPWs and all works as it should. I can send attachments.

That is the only reason I can think of as to why this PC is targeted even after fresh W11 and TB. I guess the only way to prove my theory is if somebody else whom has gone straight from OTPWs to OAuth2 as I had keeping the OTPWs still in Yahoo servers or for me to uninstall TB again > go to Yahoo website > Login and delete all OTPWs and start again by reinstalling TB with OAuth2 credentials.

At the moment I have literally wasted days on this and am sick and tired of it. For the moment OTPWs still works and allows me to send attachments so I may just leave well enough alone.