Email deliverability these days is heavily based on sending IP reputation, to a lesser extent domain reputation, and finally content.
Once you have ticked the core boxes of compliance - clean html in your templates, never send just HTML email but use multipart with plaintext, be sure of your MTA's RFC compliance, sign with DKIM/SPF - then reputation now strongly hinges on recipient behaviour at the largest ISPs. Marking an email as Spam is an obvious flag, but additional heuristics such as open rate, clicks, deletes etc. are used to determine your relationship with that recipient, and your reputation over time.
The most common issues these days with content-based filters come from smaller independently run installs - SpamAssassin, ASSP etc. As a legitimate sender you can usually cover inbox delivery to 90% of recipients by building a solid reputation on your IP with the large ISPs. The final 10% will be related to content, linked domains etc. at the smaller services.
A large reason for the growth in email-as-a-service platforms is the reputation component is already taken care of, and you can focus on content and engagement instead of the finicky aspects of core email delivery.
Once you have ticked the core boxes of compliance - clean html in your templates, never send just HTML email but use multipart with plaintext, be sure of your MTA's RFC compliance, sign with DKIM/SPF - then reputation now strongly hinges on recipient behaviour at the largest ISPs. Marking an email as Spam is an obvious flag, but additional heuristics such as open rate, clicks, deletes etc. are used to determine your relationship with that recipient, and your reputation over time.
The most common issues these days with content-based filters come from smaller independently run installs - SpamAssassin, ASSP etc. As a legitimate sender you can usually cover inbox delivery to 90% of recipients by building a solid reputation on your IP with the large ISPs. The final 10% will be related to content, linked domains etc. at the smaller services.
A large reason for the growth in email-as-a-service platforms is the reputation component is already taken care of, and you can focus on content and engagement instead of the finicky aspects of core email delivery.