efix, the part of the fax package that converts incoming files to fax format, doesn't produce a TIFF with valid X dimension, nor valid dpi values. This can be cured by recompiling with the compiler set to "-ansi". I can't explain this, but it cured my problem (not getting valid TIFF files that could be seen by "fax view" or by any program that could read TIFF files. The compiler used was standard gcc-2.7.2.3 and optimization 0 (-O0). Haven't tried anything else.
the version of efax in RawHide (and hence Red Hat 6.0) does not have this problem.