As a follow up to my post about Mail going offline, you can try the following.
Go to System Preferences>Network, select your WiFi in the left column, then click on Advanced in the lower right hand corner. Then highlight each of the wireless networks you have joined, and remove them by selecting the minus sign under the list of network names. Then turn WiFi off. Then go into keychain access and remove all of the keychain entries for each of those networks. Then turn WiFi back on, and rejoin your network from scratch.
This has worked for me, although the results for this have been mixed among users.