[MlMt] IMAP Port logic

Rainer Frädrich mail at raifra.de
Wed Aug 18 04:27:30 EDT 2021


it's possible to do TLS on port 143, but the IMAP server must be capable of understanding the STARTTLS command (which most modern IMAP servers do). So the client does explicitly request TLS handshaking by using the STARTTLS command when connected to port 143. RFC3501 has more on this.

When the client connects on port 993 instead, TLS handshaking starts right away ("implicit TLS"). For more info on this see RFC8314.

Bye, Rainer

On 18 Aug 2021, at 10:08, Marc ARC wrote:

> Hello,
> Following a close look at our mailserver due to an “attack”, we want to better understand how MM decides which port to use to open a connection.
> Port 143 – this is the default IMAP non-encrypted port;
> Port 993 – this is the port you need to use if you want to connect using IMAP securely
> We noticed that although “Require SSL” is selected, MM tries to connect on port 143, where we would have expected that port 993 would be used.
> Thanks in advance for any guidance,
> Marc
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