Email deliverability is primarily determined by two factors:
Email content: Email servers (like Gmail) do their best to ensure that emails in users’ inboxes are pertinent and relevant, and not just spam. There are several email spam words that, when found in the content of your email, will decrease its odds of ever making it to the recipient’s inboxes. Words like free, act now, Venmo, (seriously), and limited time have been identified in enough spammy emails that most email servers will automatically put them in the recipients’ spam folders or will even block their delivery outright.
Sender reputation: A sender reputation is a score that exists to inform mail servers about the quality of mail coming from your domain. If spam or low-quality emails historically originate from your email address, you’ll be penalized, and future messages might not make it to inboxes as they should. Email sender reputation is tracked by numerous sources, including ISPs (like Comcast), individual email providers (like Gmail), and third-party sources (like SenderScore).