Outlook for Mac 2011 – http error when sending mail

 My Mac users when sending mail are reporting that sometimes after selecting "send" an HTTP Error pops up, the email gets placed in the drafts folder. If they open it in the drafts folder and resend it will go - my below log shows "The server cannot service this request right now. Try again later" 

What can I do to resolve this?  It had all been working nicely unsure what changed.

**My exchange server is an Exchange 2010 SP2 Rollup4 w/Mac Outlook 2011 14.2.2

[  1656109us, d         0us] Outlook Exchange Web Services: <?xml version="1.0" encoding="utf-8"?><s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"><s:Body><s:Fault><faultcode xmlns:a="http://schemas.microsoft.com/exchange/services/2006/types">a:ErrorServerBusy</faultcode><faultstring xml:lang="en-US">The server cannot service this request right now. Try again later.</faultstring><detail><e:ResponseCode xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">ErrorServerBusy</e:ResponseCode><e:Message xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">The server cannot service this request right now. Try again later.</e:Message></detail></s:Fault></s:Body></s:Envelope>

[  1656109us, d         0us] Outlook Calendar: Engine[Exchange folder ]: Error during sync:  Error code: -18500 ErrorStr: HTTP error. The server cannot fulfill the request.

[  1656114us, d         5us] Outlook Exchange Web Services: EWS: Received response on connection=0x2a538948, status=-18800(200)

[  1736637us, d         2us] Outlook Exchange Auto Configure:  Error code: -3176

[  1736637us, d         0us] Outlook Exchange Auto Configure: No _autodiscover._tcp.EuropaSports.com service found

[  1736637us, d         0us] Outlook Exchange Auto Configure: --- HTTPS endpoints

[  1736637us, d         0us] Outlook Exchange Auto Configure: Querying https://cahtcluster.[mydomain].com/autodiscover/autodiscover.xml

[  1736637us, d         0us] Outlook Exchange Web Services: