Microsoft.Exchange.WebServices.Data.ServiceResponseException: Mailbox has exceeded maximum mailbox size in the Migration Agent for Exchange (MAgE) log file (4263941)

MAGE logs the following error:

M135 Error Microsoft.Exchange.WebServices.Data.ServiceResponseException: Mailbox has exceeded maximum mailbox size.

   at Microsoft.Exchange.WebServices.Data.ServiceResponse.InternalThrowIfNecessary()

   at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest`1.Execute()

   at Microsoft.Exchange.WebServices.Data.ExchangeService.InternalCreateItems(IEnumerable`1 items, FolderId parentFolderId, Nullable`1 messageDisposition, Nullable`1 sendInvitationsMode, ServiceErrorHandling errorHandling)

   at Microsoft.Exchange.WebServices.Data.ExchangeService.CreateItem(Item item, FolderId parentFolderId, Nullable`1 messageDisposition, Nullable`1 sendInvitationsMode)

   at Microsoft.Exchange.WebServices.Data.Item.InternalCreate(FolderId parentFolderId, Nullable`1 messageDisposition, Nullable`1 sendInvitationsMode)

   at Microsoft.Exchange.WebServices.Data.Item.Save(WellKnownFolderName parentFolderName)

   at Quest.MigrationManager.Exchange.MigrationConnectors.Ews.EwsPostAssist.PostMessage(String subject, String body, String from, Boolean isAssociated)

   at Dell.MigrationManagerForExchange.Shared.Switching.CpuuHelper.PutSwitchMessage(IPostAssist postAssist, CpuuMessage cpuuMessage)

   at Dell.MigrationManagerForExchange.Shared.Switching.BaseSwitcher.Switch()

   at Quest.MigrationManager.Exchange.Agent.Jobs.DisJob.Switch()

   at Quest.MigrationManager.Exchange.Agent.Jobs.DisJob.SwitchAndSetTimeStamp()

   at Quest.MigrationManager.Exchange.Agent.Jobs.DisJob.SwitchIfRequired()

   at Quest.MigrationManager.Exchange.Agent.Jobs.DisJob.AfterSyncActions()

   at Quest.MigrationManager.Exchange.Agent.Jobs.DisJob.AfterSync()

   at Quest.MigrationManager.Exchange.Agent.Jobs.DisPlusO365JobBase.RunJob()

2018-01-29 09:34:35.0826 Px11FC Tx29 A26 C3 M135 Trace Saving mailbox stage. Mailbox ID: 135, collection ID: 3, new mailbox stage: Failed, expected mailbox stage: ReadyToSwitch, from ThreadHelper:ThreadStart->ExecutionContext:Run->ExecutionContext:Run->ExecutionContext:RunInternal->ThreadHelper:ThreadStart_Context->QThreadManager:RunTask->QWorkTask:Run->DisPlusO365JobBase:RunJob->DisBaseJob:RunJobHandleException->StageToggle:set_MailboxStage->StageToggle:Save->SqlMailBoxStageStorage:Save