Via SSMS I have configured Database Mail profile and account.

I have started the service with: sysmail_start_sp;

It starts, I test an email which remains in unsent items; and the following error logs to the Database Mail Log; any help on below error:

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28/08/2015 2:05:54 PM

DatabaseMail process is started

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28/08/2015 2:05:54 PM

Log Database Mail (Database Mail Log)

Log ID 244
Process ID 10004
Last Modified 28/08/2015 2:05:54 PM
Last Modified By NT AUTHORITY\SYSTEM

Message
1) Exception Information
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Exception Type: Microsoft.SqlServer.Management.SqlIMail.Server.Common.BaseException
Message: The Transaction not longer valid.
Data: System.Collections.ListDictionaryInternal
TargetSite: Void ValidateConnectionAndTransaction()
HelpLink: NULL
Source: DatabaseMailEngine

StackTrace Information
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   at Microsoft.SqlServer.Management.SqlIMail.Server.DataAccess.ConnectionManager.ValidateConnectionAndTransaction()
   at Microsoft.SqlServer.Management.SqlIMail.Server.DataAccess.ConnectionManager.RollbackTransaction()
   at Microsoft.SqlServer.Management.SqlIMail.IMailProcess.QueueItemProcesser.GetDataFromQueue(DataAccessAdapter da, Int32 lifetimeMinimumSec)
   at Microsoft.SqlServer.Management.SqlIMail.IMailProcess.QueueItemProcesser.ProcessQueueItems(String dbName, String dbServerName, Int32 lifetimeMinimumSec, LogLevel loggingLevel, Byte[] encryptionKey, Int32 connectionTimeout)