Following inputs came to me from Nicolas Wyss from MiroNet:

The labels of the radio buttons in the "Security" section of the mail module are somewhat confusing.

SMTP (no encryption)
SMTP over SLL (SMTPS, SSL v3)
SMTP over TLS (extend SMTP, TLS 1.0)

The following labels would make more sense:

Example.:

Connection Security:

no encryption
use SSL/TLS
use STARTTLS

The following SMTP port/encryption combinations are common:

Port 25 - no encryption or STARTTLS
Port 465 - SSL/TLS
Port 587 - STARTTLS <--- recommended variant

see https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol