After scanning our webserver, I've got this message:
The remote SMTP service, support the user of the 'STARTTLS' command to switch from a plaintext to an encrypted communications channel.
As I cannot find many information about this on the internet I've decided to put it here. Should I disable STARTTLS? Or why do I get this security-warning?
TLS is a standard protocol to encrypt traffic in order to protect against e.g. man-in-the-middle attacks.
STARTTLS is how your mailserver informs the world that it allows secure transmissions. This is also standard. For a mailserver, it can also be used to verify that the sender of the email is someone authorised to use that mailserver.
Removing it will, in other words, not increase your security.
I am not sure how you managed to not find any information about it; a google or duckduckgo search on "SMTP STARTTLS" gave me a huge list of pages with information - which, by the way, should also be available in the documentation for your mailserver software.
