Q: Yosemite 10.10.5 Mail Issues

Try this from another user:

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.

Try the Connection Doctor >  Use Mail Connection Doctor - Apple Support

When Mail is not responding, note the exact time: hour, minute, second.  

These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.

Launch the Console application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

The title of the Console window should be All Messages. If it isn't, select

          SYSTEM LOG QUERIES ▹ All Messages

from the log list on the left. If you don't see that list, select

          View ▹ Show Log List

from the menu bar at the top of the screen.

Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.

Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.

Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.

The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.

Please don't indiscriminately dump thousands of lines from the log into this discussion.

Please don't post screenshots of log messages—post the text.

Some private information, such as your name, may appear in the log. Anonymize before posting.

When you post the log extract, you might see an error message on the web page: "You have included content in your post that is not permitted," or "The message contains invalid characters." That's a bug in the forum software. Please post the text on Pastebin, then post a link here to the page you created.