DefaultPort is a Published property that indicates the default port number to use for listener threads in the SMTP server. The value in DefaultPort is assigned as the port number for Bindings added to the server. 

The default value for DefaultPort is IdPORT_SMTP. 

The value in DefaultPort may be updated at runtime when the value in the UseTLS property is changed, and the server is not already Active. The value in IdPORT_ssmtp is assigned to DefaultPort when Transport Layer Security is implied, required, or explicitly assigned for the IOHandlers in client contexts. The value in IdPORT_SMTP is assigned to DefaultPort when Transport Layer Security is not supported.

UseTLS TIdUseTLS IdPORT_SMTP IdPORT_ssmtp