If its internal network traffic only, you can just use stalwart and add the DNS records to your network so that stuff can use SMTP.

Otherwise, (also with stalwart or your preferred software), you will want to use an SMTP relay so that outgoing messages will be delivered without the hassle of getting your domain reputational enough for sent messages not getting junked by the big email providers.

I’m extra lazy and haven’t set this up, so I just have a public mail server that only receives messages because my outgoing SMTP port is blocked by my ISP.