Some users have reported that Mail.app fails to retrieve all mail or certain messages after the update to Mac OS X 10.6.3. Mail.app may also stop automatically checking for new mail (i.e. it will only receive mail when the check mail button is clicked or the app is quit then re-launched.
Re-apply Mac OS X 10.6.3 Combo Update. Some users have experienced success in resolving this issue by using what is perhaps the trustiest post-update-issue routine: re-applying the Mac OS X 10.6.3 update. Simply download the 785 MB file and run the installer.
Change DNS servers. Failing the aforementioned, try changing your systems’s DNS servers to those offered by OpenDNS. To do so,
- Open System Preferences and click “Network”
- Select your connection method (AirPort, Ethernet, etc.)
- Enter the following items in the “DNS Server” field: 126.96.36.199, 188.8.131.52
- Click Apply