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I'm using thunderbird where I can check the option whether I will use SSL/TLS or not. However in my company's server, SSL/TLS is enabled both on POP3 and SMTP.

Is it possible to disable SSL/TLS on the client side? If so, what's the advantage/disadvantage of doing it?

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There are different protocols for retrieving the mail (POP3, IMAP) and for sending mail (SMTP). All three of these protocols can have different flavors regarding the use of TLS: no TLS at all (by default port 25 or 587 SMTP, port 110 POP3, port 143 IMAP), upgrading a plain connection to TLS using a special command like STARTTLS (SMTP, IMAP) or STLS (POP3) or using TLS from start with a different port (port 465 SMTPS, port 993 IMAPS, port 995 POP3S).

Given the variety of options there is usually a way to choose the proper option at the client side. But the choice at the client must match what the server actually supports. Many (but not all) mail servers today are configured to not accept authentication credentials over a plain (unprotected) connection and enforce the use of TLS is authentication is needed - which is practically always for retrieving the mail and in many environments also for sending the mail.

Is it possible to disable SSL/TLS on the client side?

Most modern clients default to the use of TLS. Disabling this is needed if the server does not support TLS. Disabling might be possible if the server accepts both plain and TLS protected connections but not if the server enforces the use of TLS.

If so, what's the advantage/disadvantage of doing it?

Disabling TLS removes protection and thus is not recommended if the server supports TLS. The only advantage of disabling TLS at the client is to be able to make an (insecure) connection to a server which does not support TLS at all, i.e. where having an insecure connection might be considered less of a problem than having no connection at all.

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