I have just upgraded from a e61 to a e61i. With the e61 my email worked exactly as it should: Anything deleted on my Mac was removed from my inbox on the e61, anything deleted using the e61 was automatically removed from the Macs inbox. However since upgrading to the e61i this feature is not working correctly: Emails deleted on the e61i are immediately removed from my Mac, but emails deleted on the Mac are NOT removed from the e61i.
The settings on the Mac have not changed and the settings on the e61i are identical to that of the e61.
The e61i runs the latast firmware, both phones were branded Vodafone, used on the Vodafone network and had lots of settings pre-installed. I'm in the UK if that makes any difference.
I use an unlocked E61i with my IMAP Gmail account. It takes 5-10 sec to delete a message from my phone after deleting it from Web interface of the Gmail Java client. I'm out of the UK, and do not know Mac, but you may ensure your inbox settings, especially security ports and numbers.