Few applications generate their own error codes; they usually pass
through the OS codes, sometime negating them to differentiate them from
a positive success value.
Can't speak specifically for mutt, but it is likely that EAGAIN is the
It would have been useful if you had included the exact message that
was displayed, and the sequence of operations you used to create
the error situation.
A quick perusal of the source shows no mutt specific error 11 in
the header files, but without the exact error message, one is poking
in the dark.
I'd recommend you run it with strace or truss if you can reproduce
the error, and see if a system call is returning EAGAIN.