To learn the creation of the RPM, I tried to recompile the sylpheed package (2.7.0) since sources. I so learnt the entries of the file SPEC.
To see if that worked I tried to experiment this on the last stable version of sylpheed 3.0.3. I adapted the file SPEC to this version and to the structure of BLAG and built a RPM successfully (architecture 64bit).
If some are interested, it is available there:
It turns on my PC for 2 days without crash but it is without guarantee.
Any return is welcome!