* On 06 Aug 2015, Petr Pisar wrote: 
> 
> Of course this does not explain why Thunderbird works. Maybe my dignostics is
> wrong and the message comes from different piece of software and has different
> cause. Or Thunderbird uses different encryption so NSA does not try. Or
> Thunderbird silently reopens the connection without notifying the user.

Running with this hypothesis, you might try

sudo tcpdump -v 'dst port 993 and tcp[tcpflags] & (tcp-rst|tcp-syn) != 0'

while using each of Thunderbird and mutt, to see what's happening inside
the box during the event.  (1) Do you see resets (RST, flags -> R) while
running mutt?  (2) Do you see RST followed by SYN (flags -> S) while
running Thunderbird?

-- 
David Champion • [email protected]

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