Asko P. The hardest part about making a shared hosting platform, for me, was making sure that emails don't go to spam and are trusted by receivers.

Mark Dain Oh that's a whole can of worms. We've done a huge amount of anti-spam work at the company I work for so I can give some tips if you'd like; setup DMARC for diagnostics. Have correct SPF records. W3C validate your HTML, it matters. Always send a text/plain variant. Don't base64 encode URLs. DKIM sign your emails if your MTA supports that. Valid reverse DNS is a must (both on v4 and v6) and finally, if you have various mail services listening on those ports, they should greet with the domain. Hard to explain. SMTP says HELO shadywebsite.com when you connect.