Thanks for sharing your insights, Till.
I like Amavis, too, and I have no desire to replace it.
The server has only 1GB of physical memory, which, I realize, is hardly sufficient for most environments, but this is a development server on which we would prefer not to spend more money than absolutely necessary.
We do use custom SpamAssassin rule-sets; it makes sense that this would contribute to higher memory usage.
I have changed the maximum number of concurrent Amavisd-new processes from the default (2) to 1.
For anyone else who is interested in making the same change, this is how it's done:
# The number here MUST match the amavis configuration value.
amavis unix - - - - 1 smtp
All of this said, I'm still not sure how to prevent the amavis issue that I described in my second post. Any ideas there, Till?