Several users have reported an issue in which connectivity to Microsoft Exchange servers fails after the update to iPhone OS 2.1.
Per Apple Discussions poster Jim Little:
"Since upgrading to the 2.1 iPhone software this morning, Exchange no longer works. I got an "the connection to the server failed" error. I tried reentering my password first, no luck. Then I deleted the account under Settings -> Mail, Contacts, Calendars on the iPhone, and created a new account and reentered my Exchange server settings. I got two 'unable to verify certificate' errors, then it said 'unable to connect to server'."
Users have attempted a number of traditional fixes, such as deleting and reestablishing the Exchange account and restoring the iPhone, to no avail.
Hard reset Some users have found that performing a hard reset of the iPhone resolves this issue. This can be accomplished by holding the home and sleep/wake buttons simultaneously until the screen turns off, then the white Apple logo reappears.
Delete domain In addition, some users have found that deleting domain field under Exchange account settings resolves the issue (In Settings > Mail).
Set no limit for items Finally, some users have foudn that setting no limit for contacts and calendars resolves the problem. Go to Settings> Mail, Contacts, Calendars, select your account select, thene select Mail days to sync> No Limit.
Next, go to Settings> Mail, Contacts, Calendars> then go to Calendar> Sync> All Events.