Well, ya know... I completely missed that, thanks.
Yes it appears that one needs to configure the appropriate driver.
Please ignore this portion of my last post:
"The RoundCube plug-in selects the appropriate driver for you. " ( not true!!)
Sorry 'bout that.
After looking at the hMailServer archives, I would speculate that:
driver hmail_sql_5_1 is for hMailServer 5.1 and 5.1.x builds
driver hmail_sql_5_2 is for hMailServer 5.2 builds
driver hmail_sql_5_201 is for hMailServer 5.2.1 builds
driver hmail_sql_5_300 is for hMailServer 5.3 builds
Begs the question on what to use for hMailServer 5.2.2.
But Rosali would need to weigh in as the authoritative source.