Q: when i go to my mail this is what pops up... Mail needs to repair its info about your mailboxes   Click quit then open mail . Mail will import your messages when you open it   But when I click quit and reopen it does the same thing over and over

Quit Mail. Force quit if necessary.

Back up all data before proceeding.

Triple-click anywhere in the line below on this page to select it:

~/Library/Mail/V2/MailData

Copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select

          Go ▹ Go to Folder

from the menu bar. Paste into the box that opens by pressing command-V, then press return.

A folder window will open. Inside it there should be files with names as follows:

          Envelope Index

          ExternalUpdates.storedata

Move those files to the Desktop, leaving the window open. Other files in the folder may have longer names that begin as above. Move those files, if any, to the Trash.

Relaunch Mail. It should prompt you to re-import your messages. You may get a warning that the index is damaged and that Mail has to quit. Click OK. Typically, the process takes a few minutes, but it may take hours if you have gigantic mailboxes. In that case, you may be able to speed things up by temporarily adding your home folder to the Privacy list in the Spotlight preference pane. Remove it when Mail has finished importing.

If the import operation stalls or fails with an error message, quit Mail, delete the two new index files (keeping the original ones that you moved to the Desktop), and try again. According to many reports, the second attempt may succeed after the first one has failed.

Test. If Mail now works as expected, you can delete the files you moved to the Desktop. Otherwise, post your results.

Thanks Linc. I wasn't the OP but had the exact same issue and your advice worked first time - much appreciated.

Incidentally, it started in Yosemite when for no apparent reason the mail app would launch but no mail screen would be visible, and I couldn't even access mail accounts/icloud via system prefs. Loaded El Cap, tried to launch mail but got the exact same message as the OP, and the endless cycle began.

Very grateful for your post.

Cheers, Brian

Thank you Linc:  It worked for me.  What a lifesaver.  I copied the entire Maildata folder over to a partition on my drive just in case.  I did not have to use it.  You're the man.