OS X :: Large Number Of Incoming Message - Download Speed Goes To Zero

I'm using a 2.4GHz Intel Core 2 Duo imac with 3GB memory running OS 10.5.8. I've been using mac mail at home, work and on the move for years and have only ever had a couple of minor problems with it. However last Friday, after using it normally in the office, I went out, leaving the machine to sleep. When I came back my mail wouldn't download past message number 7 and it said there were 400+ emails to download when it should have been maximum 10. I tried to shut down mail but every time I tried to shut it down it wouldn't shut down.

I had to 'force quit' even though 'force quit' didn't say 'not responding' alongside mail. When I reopened mail the number of 'new' messages dropped to only 170 odd, but still I can't get past the 10th email and the download speed goes to zero. Also I can't delete some existing messages in my inbox. They go light grey but don't delete. I am able to use mail as normal on my home mac which uses mail with the same account settings as my office mac. I can also access my emails via my iPhone and webmail.