Crisis averted!
Through a process of compacting and then replacing the mail database design with OpenNTF 1.7, then compacting again and replacing the design with OpenNTF 1.7b, all seems well now. The Reply and Forward errors disappeared and the mail databases are smaller. Hmmm…
I use a couple of PCs with Notes (one of the real advantages of replication) and it may well have ben that my "challenges" were with the sequencing of replication while replacing the mail database design.
Anyway, all is well now. Sorry if I got anyone worked up.
Cheers1