I have occasionally been having trouble with emails erroring (error: -108 Not enough memory) and then being moved to the Outbox folder. I also hear the 'Mailbox sync error' chime when this happens. The email will refuse to send until I quit and reopen Outlook, after which it sends the email immediately. 

From browsing various postings regarding the 'Not enough memory' error code it sounds like the problem is most likely not 'memory' related at all, but more likely an issue somewhere else. In an effort to resolve this I rebuilt the Outlook database, and also checked the Outlook 'Reading/Security' preferences as suggested by a few posts, although I failed to understand what 'downloading pictures from the internet' had to do with sending emails.  Nothing seems to be resolving the problem. I am sending emails throughout the day, but this seems to happen randomly.... sometimes not at all for an entire day, yet on other days it happens numerous times.  Many of the emails do not necessarily have any attachments, so I can't attribute it to that. It also occurred to me that it might only be happening when 'replying' to emails. I will have to continue to track occurrences and see if that is truly the case.

I am using Outlook for Mac 2011 v14.6.0, running on an iMac 3.5GHz i7 with 16Gb of RAM and plenty of hard drive space available.  My operating system is 10.10.3 (Yosemite).  

Every time this occurs I have checked the system.log in Console to see what is going on and I see the same thing every time:

May  2 09:56:28 mac-0c4de9c778b6.signature-graphics.com CoreServicesUIAgent[298]: unexpected message <OS_xpc_error: <error: 0x7fff72864c60> { count = 1, contents =

"XPCErrorDescription" => <string: 0x7fff72864f70> { length = 18, contents = "Connection invalid" }

}>

May  2 09:56:30 mac-0c4de9c778b6 Microsoft Outlook[24708]: objc[24708]: Class OnClickLinkAction is implemented in both /Applications/Microsoft Office 2011/Office/OutlookLegacy.framework/Versions/14/OutlookLegacy and /Applications/Microsoft Office 2011/Microsoft Outlook.app/Contents/MacOS/Microsoft Outlook. One of the two will be used. Which one is undefined.

May  2 09:56:30 mac-0c4de9c778b6 Microsoft Outlook[24708]: objc[24708]: Class ERExtraKeyedTableColumn is implemented in both /Applications/Microsoft Office 2011/Office/OutlookPaletteItems.framework/Versions/14/OutlookPaletteItems and /Applications/Microsoft Office 2011/Microsoft Outlook.app/Contents/MacOS/Microsoft Outlook. One of the two will be used. Which one is undefined.

May  2 09:56:31 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: WARNING: The Gestalt selector gestaltSystemVersion is returning 10.9.3 instead of 10.10.3. Use NSProcessInfo's operatingSystemVersion property to get correct system version number.

Call location:

May  2 09:56:31 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: 0   CarbonCore                          0x9a671291 ___Gestalt_SystemVersion_block_invoke + 135

May  2 09:56:31 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: 1   libdispatch.dylib                   0x90009130 _dispatch_client_callout + 50

May  2 09:56:31 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: 2   libdispatch.dylib                   0x900090b5 dispatch_once_f + 251

May  2 09:56:31 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: 3   libdispatch.dylib                   0x9000a0d8 dispatch_once + 31

May  2 09:56:31 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: 4   CarbonCore                          0x9a5ea69d _Gestalt_SystemVersion + 1050

May  2 09:56:31 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: 5   CarbonCore                          0x9a5e97c0 Gestalt + 150

May  2 09:56:31 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: 6   OutlookCore                         0x017d821b _ZN6carbon22HasSnowLeopardOrBetterEv + 45

May  2 09:56:31 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: OTAtomicAdd32 is deprecated and will be removed soon.  Please stop using it.

May  2 09:56:32 mac-0c4de9c778b6.signature-graphics.com Microsoft Outlook[24708]: Using your own bundle identifier as an NSUserDefaults suite name does not make sense and will not work. Break on _NSUserDefaults_Log_Nonsensical_Suites to find this

I am not knowledgeable enough to be able to determine what the system.log is telling me, if anything, and was hoping someone out there has had a similar problem, or can at least decipher the system.log.   I would appreciate some help/suggestions.