Outlook can't send mails with large attached files > 20 MB

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ASP.NET

C#3.5

Outlook

I have a SBS2013 with several Windows 7 clients running Outlook 2013.
For some reason the Outlook 2013 clients are not able to sent out mails with attacments larger than 20 MB even though Exchange 2013 seems to be configured for this??
 
 
Any helpful feedback is highly appreciated.. 

What I have tried:

if (email.Attachments.Count > 0)
{
for (int i = 1; i < email.Attachments.Count + 1; i++)
{
string attpath = System.Configuration.ConfigurationManager.AppSettings["FileAttachment"] + email.Attachments[i].FileName;
email.Attachments[i].SaveAsFile(attpath);
oMsg.Attachments.Add(email);
email.UnRead = true;
File.Delete(System.Configuration.ConfigurationManager.AppSettings["FileAttachment"] + email.Attachments[i].FileName);
}
}