Outlook test "Send test e-mail message" fails if Outgoing server port is set to 26 or 587 and Type of encrypted connection is "None":
530 5.7.0 Must issue a STARTTLS command first
'smtpd_enforce_tls=yes' is set by default in Postfix.
smtpd_enforce_tls=yes -> smtpd_enforce_tls=no
# According to RFC 2487 this MUST NOT be applied in case of a publicly-referenced Postfix SMTP server. This option is off by default and should only seldom be used. This feature is available in Postfix 2.2 and later. With Postfix 2.3 and later use smtpd_tls_security_level instead.
smtpd_tls_security_level = may
# Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not require that clients use TLS encryption.
smtpd_tls_security_level = none
# TLS will not be used.
# When a non-empty value is specified in smtpd_tls_security_level, this overrides the obsolete parameters smtpd_use_tls and smtpd_enforce_tls
Then, apply Postfix changes:
# service postfix reload
The description of Postfix options can be found in the following links: