[exim] nice / lowering exim priority

Hello,

I am curious if there is a way to lower exim's priority while it is
being run. I had a situation where having been blocked by Yahoo I
ended up with some 8K emails in queue. This would be all right but my
machine so far is humble Pentium III 866 with 512 MB RAM. So while
watching the CPU, I noticed that all day long it was being used at
100% (no idle time). I have dealt with the queue problem temporarily
but I wonder whether in situations like this there is a way to lower
exim thread's priority to leave some cycles for other tasks?

I have never done it before. I tried nice -n 15 mailnull (mailnull is
the user exim is run as) but this didn't work. I did not find out how
to do it properly but I also wonder if lowering priority may have some
serious side effects?

Many thanks for your advice, not least for your patience with me :)

--
Zbigniew Szalbot