[angelmb]

I wonder… sometimes while Mail is trying to get mail or stuff like that, OS X becomes unresponsive, I find it too weird to even ask if nowadays an app like that would still be single threaded, but since I am far from being a byte coder well… I guess there are lot of people over here with knowledge enough to give a proper reply… 

Oh, that is a G3 with 896 MB RAM running Tiger.

[mdc]

This may be way off the mark but on 10.5 in Activity Monitor my Mail.app is using 13 threads.

[Chuckit]

If the entire interface is rendered unresponsive, it wouldn't have anything to do with how many threads are in a given app, because different apps are even more separate than different threads. Temporary interface lockups are usually caused by lack of RAM, lack of disk space, or both. In your case, I would guess that more RAM would probably help. It could be that your processor is getting pinned, because that is a pretty low-end system for Tiger, but that seems a bit less likely to me than Mail just gobbling up oodles of memory.