I just moved to the HTC Thunderbolt from a BlackBerry. Although gmail supports email push using IMAP, I wanted to use my own email accounts on my own Linux server.
K-9 Mail supports IMAP push. By using a webmail (squirrelmail) server-side filter I send email marked by spamassassin with [SPAM] in the subject to a Spam folder. K-9 Mail notifies me of mail in the INBOX, and I can check the Spam folder for mail incorrectly marked [SPAM] at my convenience.
Unfortunately no spam filter seems perfect, but this method has greatly reduced the amount of spam in the INBOX and occaissionally I find a real message in the Spam folder.
Now my Android email works as well as my BlackBerry email did. I receive email in a few seconds. I have been very pleased with K-9 Mail.
Many hosting accounts support IMAP, webmail, and spamassassin, so this setup should be possible if you do not have a dedicated Linux server.
The HTC Thunderbolt with 4G LTE is fast. Using the HTC Sync software on a fast Windows 7 PC with the USB connected Internet Connection Mode, I measured 9.041 mbps download and 5.574 mbps upload in dslreports.com java speed test.