Replication Scenario

The error message can be found under 'EmailServices.log'

<![LOG[Error during async postback. A potentially dangerous Request.Form value was detected from the client (ctl00$cph1$MailParserMailboxPassword="r4pV<nJb3RgqX2x"). at    at System.Web.HttpRequest.ValidateString(String value, String collectionKey, RequestValidationSource requestCollection)
   at System.Web.HttpValueCollection.EnsureKeyValidated(String key)
   at System.Web.HttpValueCollection.Get(String name)
   at System.Web.UI.WebControls.TextBox.LoadPostData(String postDataKey, NameValueCollection postCollection)
   at System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)]LOG]!><time="13:55:51.000+240" date="4-15-2020" component="ScriptManager1_AsyncPostBackError" context="" type="1" thread="4" file="Website">
<![LOG[Unhandled Exception https://xxxxxxxx/esd/admin/Settings_Email.aspx A potentially dangerous Request.Form value was detected from the client (ctl00$cph1$MailParserMailboxPassword="r4pV<nJb3RgqX2x").    at System.Web.HttpRequest.ValidateString(String value, String collectionKey, RequestValidationSource requestCollection)
   at System.Web.HttpValueCollection.EnsureKeyValidated(String key)
   at System.Web.HttpValueCollection.Get(String name)
   at System.Web.UI.WebControls.TextBox.LoadPostData(String postDataKey, NameValueCollection postCollection)
   at System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   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)]LOG]!><time="13:55:51.000+240" date="4-15-2020" component="Applicati

Workaround

Try to avoid using above mentioned character in your account. (Underscore '_' and 'Exclamation mark '!' are supported)

Resolution

The bug will be fixed in future release (The time reported from version 2019 R1)