To get an email delivered to an inbox can sometimes be challenging and is often dependent not only on how the data was collected and the content of the email but also the type of email address.
In general there are two distinct groups. Business to business (B2B) and Business to Consumer (B2C).
Generally a B2B email address use the company name as part of the email domain, for example the email address would be @companyname.com. B2C email addresses are normally supplied and run by one of the hundreds of ISP’s that are available worldwide. Their email domain is that of the ISP such as @gmail.com or @outlook.com. The obstacles on the way to the inbox are vastly different.
In the B2C world, spam filtering at ISP’s is much more complexed but you have a much smaller number to contend with. However, if is much more about what you recipients think of your email rather than what your email contains. Companies such as Google and Yahoo have multi layered spam filtering systems that learn from billions of emails and users and what they want to receive. Content filtering and reputation still have a part to play at the top level but below this is user specific filtering based on behavior.
On the plus side, the ISP’s give you a number of tools to asses your delivery. They also provide feedback loops so that anyone who marks your emails as spam can be removed from future mailings. Having feedback loops setup for all ISP’s is an important step in B2C deliverability and is normally done by default by your ESP.
When delivering B2B email, each company can have its own spam filtering device and can change its own settings and whitelist/blacklist whenever they want. They might also incorporate much more traditional filtering systems that put much more weight on content filter such as those in spamassassin. This means things such as Image/text ratio, incorrect html, plain text version etc. may have a weighting on your spam score.
Unfortunately in the B2B world there is no feedback loop as such. If someone at a company complains it is likely to their system administrator who will then likely block your domain without any notification. For the administrator to give you any type of feedback you need to make it easy for them. Make sure your WHOIS record shows an email contact that they can email with the complaint and also that your abuse@ address for the domain is valid. Often system administrators will use these to try and contact you and ask to be removed before they go ahead and block your domain.
Implementing and looking after the basics will help with delivery to both groups. Make sure your email is always authenticated with SPF, DKIM and DMARC and monitor your IP and domain reputation. Any drop in reputation may indicate a listing on a blacklist. Many deliverability issues can be avoided with good data collection practices.