Looking at our event logs we see a lot of these messages. All the same and looks related to ActiveSync

Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 7/19/2021 1:26:25 PM
Event time (UTC): 7/19/2021 1:26:25 PM
Event ID: 146c8651bc6045568aef62ec62fd3db8
Event sequence: 17479
Event occurrence: 37
Event detail code: 0
 
Application information:
    Application domain: /LM/W3SVC/2/ROOT-1-132711510466399068
    Trust level: Full
    Application Virtual Path: /
    Application Path: C:\Program Files (x86)\SmarterTools\SmarterMail\MRS\
    Machine name: EC2AMAZ-TCS30IP
 
Process information:
    Process ID: 1652
    Process name: w3wp.exe
    Account name: NT AUTHORITY\NETWORK SERVICE
 
Exception information:
    Exception type: FormatException
    Exception message: The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.
   at System.Convert.FromBase64_Decode(Char* startInputPtr, Int32 inputLength, Byte* startDestPtr, Int32 destLength)
   at System.Convert.FromBase64CharPtr(Char* inputPtr, Int32 inputLength)
   at System.Convert.FromBase64String(String s)
   at SmarterMail.Web.SyncProtocols.EAS.EasHelperFunctions.HandleBasicAuth(String authHeader, UserData& user)
   at SmarterMail.Web.SyncProtocols.EAS.EasHelperFunctions.Authenticate(HttpContext context)
   at SmarterMail.Web.SyncProtocols.EAS.ActiveSyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

 
 
Request information:
    Request URL: https://mail.jm2.biz:443/Microsoft-Server-ActiveSync
    Request path: /Microsoft-Server-ActiveSync
    User host address: 107.77.208.212
    User:  
    Is authenticated: False
    Authentication Type:  
    Thread account name: NT AUTHORITY\NETWORK SERVICE
 
Thread information:
    Thread ID: 80
    Thread account name: NT AUTHORITY\NETWORK SERVICE
    Is impersonating: False
    Stack trace:    at System.Convert.FromBase64_Decode(Char* startInputPtr, Int32 inputLength, Byte* startDestPtr, Int32 destLength)
   at System.Convert.FromBase64CharPtr(Char* inputPtr, Int32 inputLength)
   at System.Convert.FromBase64String(String s)
   at SmarterMail.Web.SyncProtocols.EAS.EasHelperFunctions.HandleBasicAuth(String authHeader, UserData& user)
   at SmarterMail.Web.SyncProtocols.EAS.EasHelperFunctions.Authenticate(HttpContext context)
   at SmarterMail.Web.SyncProtocols.EAS.ActiveSyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
 
 
Custom event details: