When you upgrade to OS X Mavericks, a number of built-in applications will be updated to their latest versions in order to support the various features of the new operating system. One of these is Apple's Mail application, and when upgraded the program will need to update the Mail database in order for its new features to be properly used.
When this happens, you will see a "Welcome to Mail" window appear that instructs you to either continue and download Mail messages, or quit and perform this action later.
This message should only appear the first time you launch Mail, but for some on Mavericks, this message appears every time they open the program.
If this happens to your system after upgrading, you can try a quick remedy to clear it. You might consider rebuilding mailboxes and running permissions fix routines, but these will not touch the preference files responsible for the message. Instead, perform the following steps:
- Open Mail and click Continue when the welcome message pops up.
- Go to the programs preferences in the Mail menu.
- Select the Accounts section.
- Select your first account, then uncheck "Enable this account" in the "Account Information" tab; repeat for all accounts.
- Quit and relaunch Mail.
- Repeat this process, but this time enable the accounts.
When this is finished, Mail's preferences should be updated accordingly, and subsequent launches of Mail should no longer show the welcome message.