what does the output of:
netstat -n | grep :587
run as root, show you? the -p will give the program and pid.
> Date: Wednesday, July 11, 2018 21:51:09 +0000
> From: Larry Rosenman <larryrtx at gmail.com>
>
> Yep, you (probably) need to configure openSMTPD to listen on 587
>
> (I run exim, so I can't help with that).