It seems the getaddrinfo call on line 86 of libmailutils/base/hostname.c is somehow broken by this flag, though this is far outside my wheelhouse and I do not really understand what the flag does in detail or how it would possibly affect things.
I too am seeing this issue on a freshly installed unstable system. I was
initially unable to reproduce the issue on a copy of mailutils compiled
manually (without the Debian scripts). After some comparison of the two
builds, I was able to finally reproduce by adding the
-fstack-protector-strong flag to CFLAGS.
