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I'm getting this:

$ sudo netstat -alpv
tcp        0      0 comp.local:35409         ey-in-f16.1e100.n:imaps ESTABLISHED 2782/thunderbird-bi
tcp        0      0 comp.local:52950         ew-in-f16.1e100.n:imaps ESTABLISHED 2782/thunderbird-bi

Why is (or why could be) Thunderbird using such strange ports, if it's configured to use the standard pop3/smpt/imap ports?

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up vote 9 down vote accepted

A TCP connection has two sides, one who initiates and one who listens. The listener, or server, must be listening on a well-known destination port, or a destination port known ahead of time.

Ports are required on the initiator or client end as well. It doesn't have to match the destination port, indeed, most operating systems make provision for an initiator to select any available unused port.

What you are looking at is the (random) source port number chosen by Thunderbird client, which is the initiator, and it is connecting to the standard imaps destination port to your email servers, which are the listeners.

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Great, thorough answer. Thank you! – Tomasz Zieliński Nov 1 '10 at 9:03

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