When applying email message filters, it should be multithreaded with configurable number of threads for lower elapsed time of applying filters, especially when running on entire folder.
Everything should be multithreaded. I have six different email account, some personal, some for work. The work email can get an average of 30000+ emails a day, and waiting on getting mail takes forever. Even clicking for a new message is tearfully slow. Seriously, the program will freeze when processing one account/folder at a time for retrieval and filters.
Computers, even so-called cheap ones have at least 4 or more cores now days. The time of single tasking Windows 3 are over.