I am having an issue (on Umbraco 7.2.8) where Contour (3.0.23-Build.20) suddenly stops sending emails and the only way to start sending mails again, is to recycle the appPool.
When the error occurs, and a form is submitted, the following gets logged:

2016-02-17 08:46:38,408 [6] INFO  umbraco.BusinessLogic.Log - [P19116/T1/D2] Redirected log call (please use Umbraco.Core.Logging.LogHelper instead of umbraco.BusinessLogic.Log) | Type: Error | User: 0 | NodeId: -1 | Comment: umbracoForms:System.Net.Mail.SmtpException: Failure sending mail. ---> System.Net.WebException: The remote name could not be resolved: 'xx'
   at System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object owner, Boolean async, IPAddress& address, Socket& abortSocket, Socket& abortSocket6)
   at System.Net.PooledStream.Activate(Object owningObject, Boolean async, GeneralAsyncDelegate asyncCallback)
   at System.Net.ConnectionPool.GetConnection(Object owningObject, GeneralAsyncDelegate asyncCallback, Int32 creationTimeout)
   at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint)
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   --- End of inner exception stack trace ---
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   at Umbraco.Forms.Core.Providers.WorkflowTypes.SendEmail.Execute(Record record, RecordEventArgs e) in f:\TeamCity\buildAgent\work\fd622d7f786e1305\Umbraco.Forms.Core.Providers\WorkflowTypes\SendEmail.cs:line 123
   at Umbraco.Forms.Core.Services.WorkflowService.ExecuteWorkflows(List`1 workflows, RecordEventArgs e) in f:\TeamCity\buildAgent\work\fd622d7f786e1305\Umbraco.Forms.Core\Services\WorkflowService\WorkflowService.cs:line 148

I have set up a test template, where I am using library.SendMail and WebMail.Send. When loading the template, I only get an email from WebMail.Send.
When loading the template, the following gets logged:

2016-02-17 08:51:41,252 [6] ERROR umbraco.library - [P19116/T1/D2] umbraco.library.SendMail: Error sending mail.
System.Net.Mail.SmtpException: Failure sending mail. ---> System.Net.WebException: The remote name could not be resolved: 'xx'
   at System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object owner, Boolean async, IPAddress& address, Socket& abortSocket, Socket& abortSocket6)
   at System.Net.PooledStream.Activate(Object owningObject, Boolean async, GeneralAsyncDelegate asyncCallback)
   at System.Net.ConnectionPool.GetConnection(Object owningObject, GeneralAsyncDelegate asyncCallback, Int32 creationTimeout)
   at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint)
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   --- End of inner exception stack trace ---
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   at umbraco.library.SendMail(String FromMail, String ToMail, String Subject, String Body, Boolean IsHtml)

I don't know where the remote name "xx" comes from. In my config I have defined:

<network host="smtp.hnext.lan" port="25" />

This host and port is also used, when I am testing with WebMail.Send.

Any suggestions on how to fix this issue?