There are tons of things which you will have to keep in consideration to get higher inbox ratio.
Reaching inbox is no way simpler than seo. By that I mean there is no certain method which can guarantee 100 percent results.
And different people may/will have different tips and methods.
I can share the few i know
1- The ip used for mail sending software should be clean as distilled water.
2- the content may have affiliate links but should stay miles away from negative keywords. Example- lottery, gambling, *ex, di*k etc.
3- should not have too many images (this makes it looks spammy)
4- start with low volumes per day and gradually increase the daily email quota. This is called warming up.
5- should have the text which says "To Unsubscribe click here"
6- Get your email list scrubbed before shooting on the email list. Reason when there is low or zero bounces, email filters think u are a clean email marketer.
7- avoid promotional keywords example- limited time offer, 90% discount etc