Once you’ve established the issue in your mail server, use something like the following to clear it out.

 
for i in `exiqgrep -i -f nobody`; do exim -Mrm $i; done && for i in `exiqgrep -i -f "^<>$"`; do exim -Mrm $i; done && for i in `exiqgrep -i -o 59200`; do exim -Mrm $i; done

There’s a reason I’ve named this kill it with fire.