Setting up a mail server is somewhat intricate as admins added hurdles to cut spam. This was on HN a while back and is good stuff https://www.c0ffee.net/blog/m

Setting up a mail server is somewhat intricate as admins added hurdles to cut spam. This was on HN a while back and is good stuff https://www.c0ffee.net/blog/mail-server-guide https://www.c0ffee.net/blog/mail-server-guide SPF, DKIM, DMARC all need to be set up. Check to see if the IP is any RBLs. Time also helps as the major MXers "learn" you are a real mail server.