Here are some of the changes I've done to get it to work:
double check the linux input is enabled in the inputs config file.
In the inputs config file, change the interval to a longer time than 5 min. I've set ours to 900 (15 min)
Turn caching off and have it delete mail after getting. You may not be able to do this, but it works almost all the time when I've told it to delete. I believe this is what has gotten it to work for me more than anything else.
If most of your emails are rich text, try adding text/rtf to the mime types.
change the rest timeout to 60 seconds
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