Siemens SIMATIC S7-1500 Function Manual page 109

Open User Communication
6.11 Secure Open User Communication
Process for establishing a secure connection to the mail server
You can choose between two processes for establishing the secure connection to the mail
server:
● SMTPS: The client attempts to immediately establish a TLS connection to the mail server
("handshake" process). If the mail server does not support TLS, then no connection is
established.
● STARTTLS: Client establishes a TCP connection to the mail server. The client sends a
request to "upgrade" the existing connection to a secure TLC connection over the TCP
connection. If the mail server supports TLS, the client sends the command to establish a
secure connection. The mail server uses the SMTP command "STARTTLS" to do this.
The client then establishes a secure connection to the mail server. Advantage: If the mail
server does not support TLS, client and mail server can communicate unsecured with
each other.
You use the "Remote Port" setting in the data types at the block parameter
"MAIL_ADDR_PARAM" to define which process is used for the communication.
Table 6- 6
The instruction TMAIL_C uses SMTPS only for Port 465. For all other ports STARTTLS is used.
1
According to RFC, mail servers use Ports 25 and 587 for secure connections with STARTTLS. The
2
use of other port numbers for SMTP is not RFC-compliant, successful communication with such a
mail server is not guaranteed.
108
Port numbers for the SMTPS and STARTTLS processes
Process
SMTPS:
STARTTLS
Port
465
1
Any (≠465)
2
Function Manual, 11/2019, A5E03735815-AH
Communication