I recently upgraded from Lion to Yosemite. Now using Mail 8.2 the welcome screen shows up with every launch of the program. Dis- and reenabling the IMAP account like proposed here did not fix it. What can I do?
I finally found it!
With Yosemite the available options for caching messages changed. Before there were 3 possible values for the key "CachePolicy" in Accounts.plist: "CacheAllMessages" (or missing value), "CacheNoMessages" and "CacheMessagesAsRead". Now there are only two options: "CacheAllMessages" (or missing value) and "CacheAllBodies" (meaning without attachments).
If there are Accounts with one of the deprecated options "CacheNoMessages" or "CacheMessagesAsRead" configured, the welcome screen will show until the caching option is actively changed in account settings. The other possibility is to manually remove all instances of the key "CachePolicy" from Accounts.plist.