Are you using http://www.mail-tester.com/ to test the quality of emails? Make sure you get a 10/10 score after setting up. After this you'll need to warm up the IPs on the SMTP server to make sure they don't get blacklisted (Check on https://mxtoolbox.com/). You need a good IP provider like ovh.co.uk too and I wouldn't set up an SMTP without 5 IPs at least.
Exactly what I was about to say... mail-tester is the shit... mail services suck and it's a never ending bummer battle.