To be able to send a fax by mailfax with the latest version of HylaFax (4.4.x-y and further - see the post [1] to see explained the problem you may encounter without modifications), edit the file:
nano -w /var/spool/hylafax/etc/mailfax
Comment out the following lines, like:
### /usr/bin/faxmail -f Helvetica-Bold | /usr/bin/sendfax \
### -f "$SENDER" \
### -n -d "$EXT2@`/usr/bin/expr \"$HOST\" : '\(.*\)\.fax$'`"
Underneath the above lines add:
/usr/bin/faxmail -s a4 \
-n -u uucp "$EXT2@`/usr/bin/expr \"$HOST\" : '\(.*\)\.fax$'`" \
"$SENDER"