In DefaultMessageFactory it is assumed that FIX50 is the only version that is used with FIXT. This has historic reasons.

However, we should extend the DefaultMessageFactory to also handle other FIX50-versions out of the box, i.e. SP1 and SP2.
Since this will most probably break backward compatibility, we will do it for version 1.7 only.

Workaround:
Use a custom MessageFactory.
Or add your needed message factory for the FIX50 begin string. For example:

final DefaultMessageFactory defaultMessageFactory = new DefaultMessageFactory();
defaultMessageFactory.addFactory(FixVersions.FIX50, quickfix.fix50sp2.MessageFactory.class);