Does the account have two-step verification? If it does, use an app password in place of the account password. But I think you should use OAuth2 authentication, instead of 'normal password', which will work no matter if 2FA is on the account or not. Enter the regular account password in the OAuth browser window.

https://support.mozilla.org/en-US/questions/1309883

Also note that the User Name is the part of the address before @:

https://yandex.com/support/mail/mail-clients/mozilla-thunderbird.html

Hello sfhowes, thank you for your suggestions. Unfortunately, none of them solved our problem.

I followed your last link first, deleted her account in Thunderbird and added it in exactly the way it was described on the linked page. First I tried normal password, with the result described above, sending emails is not possible. Then I changed to OAuth2, the window to type in the credentials appeared, she typed it in, copied her password over from keypass (so most likely not a typo) and still got the same result: "login failed, try again or chance password?". Username is the part before @yandex.com. When I press "chance password" button, I get the same error message, the window to type in the credentials does not reappear. I deleted the yandex.com cookie but it still will not show up again.

I also checked the cookies settings, they seem to be all fine, the browser window opened and we could type in the credentials.

She does not use two-step verification. I use normal password, works like a charm. Neither normal password nor OAuth2 works for her account, regardless of if set up her account in her or in my Thunderbird.

Do the port stay the same when using OAuth2?

Any more suggestions?

What's the antivirus? Several of them block TB connections if not properly configured. Is keepass managing the passwords in TB or just storing them as a separate app? If it's not allowing the oauth token to be saved in TB, that will cause the problem. Confirm the password by logging in via webmail, and check that there are no obsolete passwords or oauth tokens in TB Saved Passwords. You can't change a password in TB - TB only applies the password that is set with the provider.

The port assignment doesn't depend on the authentication method.

if you use "OAuth2" as auth-method then Which Exact OAuth2 Related Sites/URLs Should Be Allowed In TB's Cookie Exception List ? (please upvote my Q+A both, in above linked page, if its useful/helpful) Check "OAuth2" related sections/paragraphs. To approve a 3rd-party email-client app (for example: Thunderbird, SeaMonkey, etc) for OAuth2 based access , user have to use (email-account's webmail site access) main-password , once.

TB = Thunderbird . auth = authentication. MSP = Mail Service Provider.

imho , using "Normal Password" based auth-method ( over/through the TLS/SSL based security/encryption ) , is better than "OAuth2" (token/cookie) based auth-method. and using App-key (aka: Mail-key, etc created in MSP's webmail access site/URL ) as password in TB , is better than using the email-account's webmail site access main-password. some MSP, do not accept email account-holder's (webmail site access) main-password based IMAP/POP/SMTP access, anymore.

"StartTLS" security/encryption has various weaknesses, vulnerabilities, etc , so please try to avoid it , if your MSP uses StartTLS then push/tell them to switch/upgrade into higher strength TLS/SSL based more secure & stronger encryption/security.

enabling the master-password in TB to protect passwords , is better than not-using master-password in TB.

if you allow ALL cookie then any type of tracking cookie, session cookie, etc etc ALL type of cookies including unsafe cookies will begin to be stored inside TB , thats not secure (or Privacy-Rights friendly) practice , so specify specific site/URL in Cookie-Exception list in TB.

yes , for both auth-method "oauth2" & "normal password", etc auth-method , port # remains same : imaps port is 993 , pops port is 995 , smtps port is 465.

More/Related Info: • Disable AV/FW/SS software mail-protection OR enable+add+allow EXCEPTIONS in it & add mail-server names/address . Some AV/SS creates insecure proxy/gateway for scanning/checking emails, but there are other secure way to scan/check emails . Please make sure the option "Allow AV to Quarantine email" is selected inside TB, and make sure your AV/SS is set to "Scan all files on access". • Regain old data in Windows : by using downgrade process, or by using a Second-TB, or by using manual file-transfer, etc). • Downgrade TB in macOS : downgrade by using second-TB , upgrade/downgrade TB. • AT&T , ATT-Yahoo related settings for TB. • Yahoo , ATT-Yahoo related settings for TB. • Microsoft Hotmail/Outlook related settings for TB. • Convert MAB/mork or SQLite based Address-Book into other format , MBOX (combined)-email files into other format. • Disable auto-update or update of TB (please upvote my Q+A both, inside the linked page, if its useful/helpful).

@sfhowes

Antivirus: It is avast for both our machines. I turned it off during several login attempts, it did not solve the problem. My account runs on the same machine, in the same Thunderbird, Avast never caused any problems to me.

Keepass: It is is just saving the password and she copies it over when in need of it. I am one hundred percent certain that the password is correct. We logged in several times in to the webmail and logging into her account through Thunderbird is not a problem, just sending emails.

Password: I wanted to say that when I used OAuth2 it worked the first time and the web window appeared and asked me to enter her credentials but when pressing okay Thunderbird said wrong password in a prompt. The same prompted offered me to change the password that I just typed in, but when pressing this the OAuth2 window did not come back and I had no chance to change the password that Thunderbird stored for the account.

@atErik Thanks for the post. I did not really see any new suggestions in it though.

I can life without OAuth2 and use normal password but that that is not working as well.

I disabled Avast and have an account that works with the exact same settings, see above. Question is why this new account will not let me send emails. Neither in the or another Thunderbird on a different machine.

We both use TLS/SSL

See this topic about Avast: https://support.mozilla.org/en-US/questions/1312586

Passwords and oauth tokens can be removed from Saved Passwords in Options/Privacy & Security. After doing so, restart TB and enter the password when prompted. Also, check that cookies are allowed in TB Options/Privacy & Security, as this is necessary for OAuth2, which must be set for both the incoming and outgoing servers.

@sfhowes

I checked the cookie settings, all enabled. Deleted the passwords for her account in Privacy and Security, started windows in safe mode, disabled autostart for Avast, restarted the computer. I checked that Avast was still disabled and started thunderbird. The OAuth2 window of Yandex.com appeared, I entered the credentials, no problem. Thunderbird loaded all folders and emails correctly. I try to send an email, the OAuth2 window appears again, I entered all credential as I did before, the windows closes without an error from Yandex, the Thunderbird prompt appears, telling me that the password for smtp.yandex.com is incorrect.

During the whole time I work with my own Yandex.com account and send and receive emails with the same Thunderbird, no problem. I do understand what causes this issue.

Since the two Yandex accounts are in the same profile, I suspect the one that can't send is trying to send on the smtp that's set for the other account. Select an account in the left pane of Tools/Account Settings, then check the Outgoing Server (SMTP) in the right pane. Each account should be sending on an smtp with a matching User Name, not both sending on a Default smtp. Click Edit SMTP server... to check the User Name. Add another smtp if necessary in Outgoing Server (SMTP) at the bottom left of Account Settings, then match the account with the correct smtp.

Hi,

I checked all those settings both on my and here Thunderbird, they are all account specific and correct. As I said, receiving emails and loading up the correct folder structure is not a problem, just sending emails. Also, she gets exactly the same error on her Laptop with her Thunderbird, what makes me think that some setting in Yandex could be the problem.

I have been using Yandex and Thunderbird for a year or two (no problems) and do not know why I cannot set up her account.

:(

Update

My own account was on the default smtp, hers on a second one. I created a new smtp server with my settings and changed my smtp server from default to this new one, works like a charm for me but still does not solve the problems with her account.

Any other ideas?

Since one account works but the other doesn't, is it possible that there's some setting in Yandex that is different, e.g. IMAP or POP is not enabled for the other account? Don't overlook the possibility there's a typo in one of the settings, or even a spurious leading or trailing space in one of the entries.

I triple checked every entry in thunderbird and checked all entries in yandex that seemed to be related (not that many though). Any idea how to find out on which side (yandex or thunderbird) the problem originates from?

Try adding the account in a new profile: Help/Troubleshooting, about:profiles, in case there's some cookie or setting in the current profile that is affecting the account. Also, remove Avast (not 'turn off') while doing this.