Rated 5 out of 5 stars

Very good - I use it for "waiting for followups" to make sure I get the reply I wanted.

Be warned that if you have a big mail box (mine is 10GB+), it can take Thunderbird a long time to load - but once loaded and the indexes are sorted, it appears to be OK.

Like another poster, it would be great if it was hidden from your inbox until it was overdue.