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Yes, but the Apache works fine. My site opens in HTTPS under the new cert OK. No problem with that at all. I've only found in /etc/postfix/main.cf couple of lines referring to the SMTPD TLS Files. But those two directives have links to the files I DID NOT touch. I think those two were created by Open SSL when I was configuring Postfix. Also Postfix does work, otherwise I wouldn't be getting e-mail in Squirrel Mail.
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Hello!<br />
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I recently switched my SSL (TLS) certificate from a regular one to Let's Encrypt. I decided to get rid of my old expiring cert files, but I physically removed them, I've started to have strange issues with Thunderbird Mail. Normally after I open the application, I get a login box. And after the old cert files removal, I stopped getting the login and also stopped receiving mail. Sending was fine. Also the SMTP server was working, since in a web mail access everything worked as usual. Only after I put back files that belonged to the old cert, Thunderbird Mail started to work as usual. I don't understand the connection between my old cert files that I was using for HTTPS and Apache and Thunderbird Mail, its login and receiving mail in it.
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Hello!

I recently switched my SSL (TLS) certificate from a regular one to Let's Encrypt. I decided to get rid of my old expiring cert files, but I physically removed them, I've started to have strange issues with Thunderbird Mail. Normally after I open the application, I get a login box. And after the old cert files removal, I stopped getting the login and also stopped receiving mail. Sending was fine. Also the SMTP server was working, since in a web mail access everything worked as usual. Only after I put back files that belonged to the old cert, Thunderbird Mail started to work as usual. I don't understand the connection between my old cert files that I was using for HTTPS and Apache and Thunderbird Mail, its login and receiving mail in it.

Removing Expiring SSL (TLS) cert caused inability to receive mail in Thunderbird Mail

HTTPS is a protocol for secure communication over a computer network which is widely used on the Internet. HTTPS consists of communication over Hypertext Transfer Protocol (HTTP) within a connection encrypted by Transport Layer Security (TLS) or its predecessor, Secure Sockets Layer (SSL). The main motivation for HTTPS is authentication of the visited website and to protect the privacy and integrity of the exchanged data. HTTPS is widely used for protecting page authenticity on all types of websites, securing accounts and keeping user communications, identity and web browsing private.

SSL / HTTPS

Thunderbird is an email, newsgroup, news feed, and chat (XMPP, IRC, Twitter) client that supports POP and IMAP and LDAP address completion. The vanilla version is not a personal information manager, although the Mozilla Lightning extension adds PIM functionality. Thunderbird can manage multiple email, newsgroup, and news feed accounts and supports multiple identities within accounts. Features such as quick search, saved search folders, advanced message filtering, message grouping, and labels help manage and find messages. Thunderbird incorporates a Bayesian spam filter, a whitelist based on the included address book, and can also understand classifications by server-based filters such as SpamAssassin. Extensions allow the addition of features.