For those who see errors like this:

Cannot open serial port /dev/com18, error: No such file or directory

Notation comX only supports up to 16 devices in Cygwin.
To use up to 64 devices on Cygwin one should use ttySx notation.

Please notice difference in numbers:

/dev/ttyS0 == /dev/com1
/dev/ttyS1 == /dev/com2

etc

Please do above mentioned changes in your smsd.conf file.

Related links:
http://comments.gmane.org/gmane.os.cygwin/136963
http://cygwin.com/cygwin-ug-net/using-specialnames.html#pathnames-posixdevices