Shaun J. Stuart notes that database mail apparently still requires .NET Framework 3.5:
However, I did run into one issue later on regarding database mail. I was able to configure it correctly and was even able to send a test email through SSMS without any errors. Unfortunately, the email was never delivered. Additionally, the database mail logs did not show database mail starting or attempting to send the message. I checked with my Exchange administrator and he said he never saw the SQL Server connecting to the mail server.
A check of the database mail table in msdb (sysmail_allitems) showed my messages sitting there with a status of Unsent.
After troubleshooting for a couple of hours and getting nowhere, we installed in the .Net Framework 3.5 and suddenly database mail started working.
This, hopefully, is a bug. But not getting e-mail alerts you’re expecting to receive can be a scary scenario.