I'm the co-founder and CTO of Medical Joyworks. You might be wondering - why is a medical education company building an inbox delivery tool?
We have a 150+ member international medical board (specialist doctors who peer-review our medical education content). We communicate with them both directly as individuals, and as a group via a mailing list.
We've found that some of our emails tended to end up in spam, despite their content being very much non-spammy - and even though our email domain has neutral reputation (we just don't sent out a lot of emails!); and we've got SPF, DKIM, etc. setup. We even tried to validate our emails against Spamassassin - but our scores were extremely good (-1 to -6).
We figured that instead of dealing with tools that try to guess whether your email might be spammed, we might as well test our emails against real email accounts. So, we hacked up a set of scripts to send emails from our domain to a set of Gmail, Hotmail, Outlook and Yahoo accounts, and just started playing around.
We were kinda shocked to see how seemingly trivial changes in an email's subject and body could massively affect inbox rates. In fact, Microsoft's email services (Outlook, Hotmail, and Live) were consistently sending our emails to the spam folder. Thanks to our tests scripts, we managed to adjust our emails' subjects/bodies to get 90% to 100% inbox delivery.
We've had lots of experience with dealing with emails while juggling with these scripts. Because they turned out to be so useful for us, we polished up their code, slapped on an easy to use-UI, and put out a proper side project, Vetter: https://vetter.monsoonyeti.com/