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Update to T-Bird 52.5.2, and cannot send e-Mails from any account.

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Any account I try, get the following: "Sending of the message failed. "The message could not be sent using Outgoing server (SMTP) mail.gmx.com for an unknown reason. Please verify that your Outgoing server (SMTP) settings are correct and try again."

Then: "Add Security Exception "This site attempts to identify itself with invalid information. "Unknown Identity "This certificate is not trusted because it hasn't been verified as issued by a trusted authority using a secure signature."

And the Confirm Security Exception button does nothing.

Identical inbound and outbound settings on my other PC, not yet updated to 52.2.x, and they all work.

Any account I try, get the following: "Sending of the message failed. "The message could not be sent using Outgoing server (SMTP) mail.gmx.com for an unknown reason. Please verify that your Outgoing server (SMTP) settings are correct and try again." Then: "Add Security Exception "This site attempts to identify itself with invalid information. "Unknown Identity "This certificate is not trusted because it hasn't been verified as issued by a trusted authority using a secure signature." And the Confirm Security Exception button does nothing. Identical inbound and outbound settings on my other PC, not yet updated to 52.2.x, and they all work.

Chosen solution

Thunderbird doesn't recognize the Geo Trust Inc. CA cert. However it should exist in the Thunderbird certificate store. It's possible your certificate store got corrupted. So you can try to delete the file cert8.db in your Thunderbird profile folder when Thunderbird is closed. The file will be re-created the next time Thunderbird is started. This assumes there are no personal certificates in the certificate store. If so, make sure to back them up prior to deleting the file.

https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

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Inspect the certificate - see attached screenshot for instructions.

Who is the issuer of the cert? Please post a screenshot of the Certificate Viewer window with the issuer information visible.

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Certificate from Geo Trust Inc.

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Chosen Solution

Thunderbird doesn't recognize the Geo Trust Inc. CA cert. However it should exist in the Thunderbird certificate store. It's possible your certificate store got corrupted. So you can try to delete the file cert8.db in your Thunderbird profile folder when Thunderbird is closed. The file will be re-created the next time Thunderbird is started. This assumes there are no personal certificates in the certificate store. If so, make sure to back them up prior to deleting the file.

https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

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Yes, Geo Trust Inc. CA was in the certificate store. Apparently something was amiss, though. Deleted the "cert.db" out of my T-Bird profile. Shut down the PC. Restarted. Opened T-Bird, and is asked about accepting the unrecognized cert(s) for my mail accounts that automatically check for inbounds. Gave it the Okay, inbounds came in just fine. Tried to send the "test" messages I had played with for the past three days . . . all went out from each account. Great! Thank you.