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I have been reading on how to configure sendmail as a smtps server. I have a personal rule through that I never enter anything into a configuration file unless I understand what it does. While for many aspects of what I've been reading I've been able to look up it's function I've found one that I cannot find any documentation for.

Many tutorials have a line similar to this line in their sendmail.cf file:

DAEMON_OPTIONS(`Family=inet,  Name=SMTPS-v4, Port=smtps, Modifiers=Eas)dnl

Every part of that I can look up and understand except the s modifier. I have found a couple of web pages that outline the available modifiers, none that show what the s modifier does.

I would guess based on how this documentation defines S that the lowercase s forces TLS to be enabled but not with enough confidence that I am ready to try it.

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The s flag enables the obsolete SMTP/SSL(465) protocol. It's been replaced by a combination of TLS over SMTP(25) and TLS over Submission(587).

As at January 2018 the relevant reference documentation is RFC 8314.

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