IMMSMC the early Linux had problems with some old-good programs i.e. Sendmail. New-born Linux hesitated between POSIX-BSD-Solaris semantic i.e. in file-locking, pty, network interface binding e.t.c. === Linux Something broke between versions 0.99.13 and 0.99.14 of Linux: the flock() system call gives errors. If you are running .14, you must not use flock. You can do this with -DHASFLOCK=0. Around the inclusion of bind-4.9.3 & linux libc-4.6.20, the initialization of the _res structure changed. If /etc/hosts.conf was configured as "hosts, bind" the resolver code could return "Name server failure" errors. This is supposedly fixed in later versions of libc (>= 4.6.29?), and later versions of sendmail (> 8.6.10) try to work around the problem. Some older versions (< 4.6.20?) of the libc/include files conflict with sendmail's version of cdefs.h. Deleting sendmail's version on those systems should be non-harmful, and new versions don't care. Sendmail assumes that libc has snprintf, which has been true since libc 4.7.0. If you are running an older version, you will need to use -DHASSNPRINTF=0 in the Makefile. If may be able to use -lbsd (which includes snprintf) instead of turning this off on versions of libc between 4.4.4 and 4.7.0 (snprintf improves security, so you want to use this if at all possible). NOTE ON LINUX & BIND: By default, the Makefiles for linux include header files in /usr/local/include and libraries in /usr/local/lib. If you've installed BIND on your system, the header files typically end up in the search path and you need to add "-lresolv" to the LIBS line in your Makefile. Really old versions may need to include "-l44bsd" as well (particularly if the link phase complains about missing strcasecmp, strncasecmp or strpbrk). Complaints about an undefined reference to `__dn_skipname' in domain.o are a sure sign that you need to add -lresolv to LIBS. Newer versions of linux are basically threaded BIND, so you may or may not see complaints if you accidentally mix BIND headers/libraries with virginal libc. If you have BIND headers in /usr/local/include (resolv.h, etc) you *should* be adding -lresolv to LIBS. Data structures may change and you'd be asking for a core dump.