@qxpd Possible and maybe even likely if your uninstall was incomplete. What platform are you on? How are you sending mail? Are you running your own smtp relay? If so, what are you using? Exim? Postfix? Or using third party SMTP relay service? Cuz those types of small details really do make a big difference.
As to the error message above, port 587 is the "well known" Submission port for SMTP. 127.0.0.1 is the local host's "loop back" interface which your machine favors for moving bits about whilst talking to itself. In this case: NodeBB is trying, unsuccessfully, to relay mail to the world via an SMTP server running on your local host. Is there one? If you do not know, you should not be running your own mail server as there be lots of security related dragons lurking thereabouts for even seasoned sysadmins.
Hence, unless you want to shoulder the responsibility of running your own SMTP server I'd suggest using a third party relay. Now, one thing that IS relatively simple to set up is a Null Client Relay, that ONLY accepts mail from the trusted local host and then relays that mail via SMTP protocol to your upstream internet facing smtp relay. Faster than the nodejs built in. And it is nice to have a "mail spool" in event outbound mail gets backed up for some reason. But that built-in works too. And a non issue with smaller sites. At least last time I looked at it.
Hope this helps some.