Q: os x update 10.10.4; mail won't send 6/30/15

you don't have to import message by messages,  one mbox equal to one folder. 

I wish i can go back to 10.10.3 but due to the beta locked up my photo library,  really need to get back to 10.10.4 in order to access my older photo library

Good luck to you all!

I'm experiencing the same problem. Two computers updated to 10.10.4 today, and neither can send mail any longer from several accounts. Receiving mail is fine. The computers that are still running 10.10.3 are sending mail just fine.

So far, Linc's method works: checking on "Automatically detect and maintain account settings" under the Advanced tab. I was really worried about this. Ten Gmail accounts in my Mac Mail setup kept going offline immediately after the 10.10.4 update today (7/1/2015). Is "Automatically detect and maintain account settings" a brand new setting with Mac OS X 10.10.4? This should not have happened. Many people will be perplexed.

That fixes incoming mail servers, but I am still having trouble with a few of my SMTP (outgoing) servers going offline. Something is wonky there.

The "Automatically detect and maintain account settings" does not solve this for me; it's not a new setting anyway. The "Connection Doctor" doesn't help either because it just tells me it can't reach the server and that recommends vaguely that maybe my settings or password or something are incorrect. The settings are all correct, it's just the update to 10.10.4 that's killed my access to outgoing mail. I want to downgrade 10.10.3 but the standalone installer won't let me do that.

@bsiu922

I deleted everything in the ~/Library/Mail folder on two of our machines, after having quit Mail.app.

I restarted the machines.

Launched Mail.app, which prompted me to create a new account, I did so, and I ran into exactly the same problem. I can receive but I can not send anything. Port 25 and 465 do not work at all (I see nothing in the server logs), only port 587 is working and when it connects to the server I get:

Jul  1 13:20:13 postoffice sendmail[13872]: STARTTLS=server, error: accept failed=0, SSL_error=5, errno=0, retry=-1

Jul  1 13:20:13 postoffice sendmail[13872]: t61KKCbf013872: [redacted] did not issue MAIL/EXPN/VRFY/ETRN during connection to MSA

Jul  1 13:20:13 postoffice sendmail[13873]: STARTTLS=server, error: accept failed=0, SSL_error=5, errno=0, retry=-1

Jul  1 13:20:13 postoffice sendmail[13873]: t61KKD4J013873: [redacted] did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA

Jul  1 13:20:13 postoffice sendmail[13874]: STARTTLS=server, error: accept failed=0, SSL_error=5, errno=0, retry=-1

Jul  1 13:20:13 postoffice sendmail[13874]: t61KKDF9013874: [redacted] did not issue MAIL/EXPN/VRFY/ETRN during connection to TLSMTA

Sorry bsiu922 - but this is not particularly helpful.  When you say things like: "We first backup the the V2 folder in the Mail in User/Preferences"   it would be helpful to be explicit about where these files are.   I do not manually manipulate them very often and I, for one, can't find where you mean.

I think I understand what you mean about the folders and mbox's...  but ...

Even better would be an actual solution from the company in Cupertino who apparently ...   sigh.  So fed up.

Anyone know how to downgrade to 10.10.3? I tried downloading the original Yosemite installer from App Store, but it's 10.10.4. And after reinstalling from that, I'm still stuck with no outgoing mail.

@jawkneemac

It's actually in /Users/<user_name>/Library/Mail

You can delete the entire folder and will be re-created by Mail on start up. But I've tried this and it did not help.

You can get to the folder by opening finder going to the "Go" menu and holding down the "alt" (option) key to display "Library" which is the users Library folder. In there you will find a folder called Mail.

Deleting this will a allow you to re-launch Mail.app and start over again.

WARNING!!! That is the folder that contains ALL MAIL DATA!!! Deleting it will delete you mail data.

I tried the technique my moving it to the desktop, but it still did not fix the issue for me.

@Ckovacs

You will have had to have time machine, or have made yourself an install disk form a previous version. There is no rollback function.

the v2 folder is in the /Users/Username/Library/Mail

in finder,  you can click go on finder,  and press the option key,  the Library will show up

then in Library,  go to Mail,  then you will see the V2

for downgrade back 10.10.3, I hope you have Time Machine before you upgrade cause once you open the Photos in 10.10.4  then the Library will not able to open within 10.10.3

you just need to total wipe the drive,  fresh install a 10.10.3,  then restore from your Backup,  that what i did when i hit that stupid SMTP issue when I use the 10.10.4 beta

You probably need to remove all the account too, 

also apple tech mention to remove one folder from the /Library/Containers/com.apple.mail

Still use it at your own RISK!!!!!!!!

@bsiu922

I deleted ~/Library/Containers/com.apple.mail

I also deleted the entire mail folder again. restarted the computer. started up Mail.app, ran into the same problem. I receive everything fine, but I can not send. Last tested settings were port 587 SSL using password.

I'm still getting on the server logs:

Jul  1 14:40:00 postoffice sendmail[17579]: t61LdYYb017579: client.example.com [redacted] did not issue MAIL/EXPN/VRFY/ETRN during connection to MSA

Jul  1 14:40:00 postoffice sendmail[17554]: t61Ld8Ki017554: client.example.com [redacted] did not issue MAIL/EXPN/VRFY/ETRN during connection to MSA

Sadly I have tried most of this but the issue remains. I have two mail accounts on my Mac and iPhone - one is working fine with SMTP on port 465 SSL but the other just will not send out of port 587 SSL.

Its the same on iOS 8.4 so clearly a change Apple made to do something but now I wish I could work out what.... The logs are pretty useless too.

Same problem after 10.10.4 upgrade.

Starting Mail with the SHIFT key pressed seems to work for me.

Support says that reindexes or something.

I had this problem with two accounts hosted by Google Apps: was no longer able to use the account after the 10.10.4 upgrade (interestingly, SMTP was fine, but IMAP/incoming was not). The following fixed it for me:

1. Check the "automatically detect and maintain account settings" box

2. I was using SSL/password, and I changed that to SSL/MD5

Now all is working fine. My guess is that if SMTP is broken, making that same change may help others.