Oh it totally used to be easy. You needed to know about DNS and MX records and how to configure sendmail and you were good to go (optional IMAP/POP3 retrieval too).
Now, it's SPF, DKIM, DNSSEC (needed for strict DKIM), DMARC ensure you have good certificates for secure smtp/pop3/imap and even then if you have a funky domain (like most instances do) outlook/gmail will probably reject you into spam.