Updated sendmail to 8.11 (RedHat 7 version) and now sendmail listens on port 587. Why is that? Since it is sendmail is linked to cyrus-sasl and OpenLDAP it might be because of that. (But then again - we do not use LDAP or SASL here).
It's the MSA service sendmail now provides. Quoting the release notes: sendmail implements RFC 2476 (Message Submission), e.g., it can now listen on several different ports. Use: O DaemonPortOptions=Name=MSA, Port=587, M=E to run a Message Submission Agent (MSA); this is turned on by default in m4-generated .cf files; it can be turned off with FEATURE(`no_default_msa').