Hi,
In my experience, 993 has wider compatibility and usually requires less initial configuration when using SSL on port 993 than when initiating a connection on port 143 and then upgrading to TLS, but it is true that TLS is considered to be more modern and secure, and if your system and clients support it, you can certainly configure IMAP4 to use TLS.
To summarize: 993 may have better compatibility, while 143 has better security.
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