FIX: SQL Server 2016 Database Mail does not work on a computer that does not have the .NET Framework 3.5 installed

Applies to: SQL Server 2016 DeveloperSQL Server 2016 EnterpriseSQL Server 2016 Enterprise CoreSQL Server 2016 StandardMicrosoft SQL Server 2016 Service Pack 1 

On SQL Server 2016 clicking the DB Mail executable presented this message:

I cannot find this requirement anywhere in the SQL Server 2016 documentation, yet it is clearly a requirement. DB Mail is functioning perfectly with no other changes than installing .NET 3.5.

Symptoms

Assume that you install Microsoft SQL Server 2016 on a computer that has the .NET Framework 4.6.1 installed. If the computer does not have the .NET Framework 3.5 installed, the Database Mail feature does not work correctly. For example, if you configure and send a test email message, the message is queued but is never sent.

Note This issue also affects instances that are running SQL Server 2016 SP1 CU1. This issue occurs because of a bug in SQL Server 2016 SP1 CU1 setup. The setup for SQL Server 2016 SP1 CU1 deletes the config file DatabaseMail.exe.config without replacing it with a new one. This causes the Database Mail to break in the absence of .Net framework 3.5 SP1. The fix in SQL Server 2016 SP1 CU2 is meant to address this issue and prevent the DatabaseMail.exe.config file from getting deleted from the machine after installing a CU. So if you install SQL Server 2016 SP1 + CU2, the Database Mail will not be impacted by the setup. However, if the Database Mail is broken by the installation of SQL Server 2016 SP1 CU1, you can use any one of the workarounds mentioned below.

https://dba.stackexchange.com/questions/142080/sql-server-2016-db-mail-not-sending

https://support.microsoft.com/en-ph/help/3186435/fix-sql-server-2016-database-mail-does-not-work-on-a-computer-that-doe