Dave,
Found the problem... Turns out the hard disk containing /var/opt/scalix is failing.
It's not reporting that its failing, and all but the closest scrutiny shows it to be
working OK. Heck, I even ran a full disk check on the thing and it reported all to
be happy. Replaced the disk in the wee hours of this morning and lo and behold
mail flows at a reasonable rate again.
My humble apologies for ever doubting Scalix... I will never do so again. ;-)