Hi Linc,
I am also having a horrible issue with Mail, in which I open Mail, and there is a pending outgoing message in the outbox. It opens this message, then keeps changing focus between the main mail viewer window and this outgoing mail. Incoming mail does come in, but ther eis no way to stop it from changing focus, which is every half a second or so. Here is the console log output for this behavior:
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3/26/16 11:08:04.483 AM com.apple.cts[4136]: com.apple.ical.sync.x-coredata://58B80EE1-7A39-4D62-BF93-CF4D0E6E0673/CalDAVPri ncipal/p3: scheduler_evaluate_activity told me to run this job; however, but the start time isn't for 3851 seconds. Ignoring.
3/26/16 11:08:04.644 AM accountsd[4170]: [AOSAccounts] : [MMCopyMailAliasForAccount] : Error from _AOSAccountRetrieveMailAliasInfo Error Domain=AOSErrorDomain Code=1000 "(null)"
3/26/16 11:08:04.645 AM accountsd[4170]: [AOSAccounts] : [iCloudAccountAuthorizationPlugin] -[iCloudIDAuthenticationPlugin discoverPropertiesForAccount:accountStore:options:completion:] : mailProperties was not modified for account ID: (EDITEDOUT)
3/26/16 11:08:04.675 AM com.apple.cts[4136]: com.apple.ical.sync.x-coredata://58B80EE1-7A39-4D62-BF93-CF4D0E6E0673/CalDAVPri ncipal/p3: scheduler_evaluate_activity told me to run this job; however, but the start time isn't for 3851 seconds. Ignoring.
3/26/16 11:08:06.999 AM Mail[15124]: XOAUTH2 requires user
3/26/16 11:08:06.999 AM Mail[15124]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
3/26/16 11:08:07.425 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:07.425 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:08.128 AM accountsd[4170]: AIDA Notification plugin running
3/26/16 11:08:09.349 AM Mail[15124]: XOAUTH2 requires user
3/26/16 11:08:09.349 AM Mail[15124]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
3/26/16 11:08:09.970 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:09.970 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:10.199 AM Mail[15124]: <MFPOPConnection: 0x7fa5e9fde5f0> (Disconnected) account: <MFPOPAccount: 0x7fa5e9f8f280> (EDITED OUT) pop.bizmail.yahoo.com (86B92F17-6D1C-49FE-A281-3DC740D57643) Error fetching 1 messages
3/26/16 11:08:10.395 AM Mail[15124]: XOAUTH2 requires user
3/26/16 11:08:10.395 AM Mail[15124]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
3/26/16 11:08:10.701 AM accountsd[4170]: AIDA Notification plugin running
3/26/16 11:08:10.820 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:10.820 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:11.258 AM Mail[15124]: XOAUTH2 requires user
3/26/16 11:08:11.258 AM Mail[15124]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
3/26/16 11:08:11.480 AM accountsd[4170]: AIDA Notification plugin running
3/26/16 11:08:11.658 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:11.658 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:12.102 AM Mail[15124]: XOAUTH2 requires user
3/26/16 11:08:12.102 AM Mail[15124]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
3/26/16 11:08:12.368 AM accountsd[4170]: AIDA Notification plugin running
3/26/16 11:08:12.485 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:12.486 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:13.000 AM Mail[15124]: XOAUTH2 requires user
3/26/16 11:08:13.000 AM Mail[15124]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
3/26/16 11:08:13.152 AM accountsd[4170]: AIDA Notification plugin running
3/26/16 11:08:13.427 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:13.427 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:13.847 AM Mail[15124]: XOAUTH2 requires user
3/26/16 11:08:13.847 AM Mail[15124]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
3/26/16 11:08:14.161 AM accountsd[4170]: AIDA Notification plugin running
3/26/16 11:08:14.233 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:14.233 AM Mail[15124]: No worthy mechs found
3/26/16 11:08:14.597 AM sharingd[4165]: 11:08:14.596 : SDConnectionManager:: XPC connection invalidated
3/26/16 11:08:14.725 AM accountsd[4170]: AIDA Notification plugin running
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