I understand this now. The latest version of Raspbian is based on the latest Debian Stable (codenamed Stretch). This is the OS I use as my main system, and email alerts don't work on my system either. DSLstats uses a rather old library to support email alerts, and this doesn't function properly with Debian Stretch (and maybe not with some other systems).
I've located a newer library and got this working properly in a little test program, and I'll now start building this into DSLstats. It will take a little while but hopefully not very long.