After upgrading to 10.10.4 mail hangs intermittently

Try a restart.

Do a backup, using either Time Machine or a cloning program, to ensure files/data can be recovered. Two backups are better than one.

Try setting up another admin user account to see if the same problem continues. If Back-to-My Mac is selected in System Preferences, the Guest account will not work. The intent is to see if it is specific to one account or a system wide problem. This account can be deleted later.

Isolating an issue by using another user account

If the problem is still there, try booting into the Safe Mode using your normal account. Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer. For each disable/delete, you will need to restart if you don't do them all at once.

Safe Mode - About

Safe Mode - Yosemite

I have the exact same problem (I am on a new MacBook). It was in 10.10.4 and has gotten WORSE in 10.10.5. If you hold down shift while launching Mail you can prevent this from happening. But right now with 10.10.5 it is almost 70% of the time. I had another thread that has kind of died in the new Apple support forums (which tends to make it much harder to find topics).

I have opened a case and honestly am tired of dealing with it after a few months. My basic approach is to just get rid of my MacBook and go back to an older MBA where I can run 10.9.x (where none of the Mail problems exist). I was ok waiting for a fix because this was a "new" machine, but after three months and two software releases I have to say enough.

If something as basic as Mail doesn't work on a new machine due to MacOS X bug that don't get resolved then what is the use of a MacBook?

BTW I am a professional developer and have performed all the "troubleshooting" steps. Only thing I haven't done yet is to restore from time machine. But considering this problem has existed from the clean version of OS 10.10.x on this machine, and I have tried a "Combo update" with no fix I am not sure I am willing to go through the hassle of a restore just to end of in the same place. This is clearly a software bug.

Highly frustrated.

Here is what I finally did: I gave up on Mail. I think I have same issue as you, I have a few IMAPs I use for work, and lots of messages and folder.

I just downloaded PostBox (third party app). It imported my mail settings painlessly. And it is incredibly fast on launch, checking all my e-mails. It is what Mail used to be and what you would think it should be. It is easily 10x faster than Mail in connecting and checking for messages. I have very little stick time, so can't really speak to all of its ups/downs, but it may be worth a look. Only things I miss right now is there is no clear activity display (which at this point I always leave open out of inherent fear of Mail freezing up). But it has a very cool quick move of messages by one keystroke that I use all the time for filing new messages/

And oh, obviously it does not hang or crash or keep showing all accounts offline!

The problem went away after upgrading to 10.10.5. I guess it was fixed, in my case, the problem worsen after I added a hotmail account which was recent, so perhaps 10.10.5 fixed that.

I have the same problem -- the mail app hangs during startup. The problem began when I upgraded from Mountain Lion to Yosemite. If I hold the shift key down while starting up mail, the problem does not occur. And if I leave the mail settings window open when I quit, it will start again without problems.

If I don't do that, most of the time (but not always) mail hangs. If I show the activity window, I'll see many instances of a generic "opening mailbox" (with no explanation of which mailbox (I have many accounts). You can't quit when this happens. The only way out is a force quit.

Another clue (after dozens of quits and launches of mail) -- this only occurs when I am displaying my inbox. If I select any of the separate accounts under the inbox label, it will start normally. I had thought that by going through them methodically, I could isolate the problem, but no luck. I've also tried turning on and off and then on "automatically detect and maintain account settings." That didn't do anything.

I'm guessing from this thread that there isn't much I can do except upgrade to El Capitan, but that is incompatible with several of my apps, so I can't make the jump yet.

Has anybody discovered anything new since the last post?

Thanks in advance,

Steve

I tried the solution that bthomp2420 suggested near the end of this thread:

Mail and Safari hang on quit, plus other symptoms

("Try deleting the contents of ~/Library/Caches/com.apple.iCloudHelper and then kill the com.apple.iCloudHelper process in the activity monitor.")

So far, that seems to have worked.