Deliverability - the art of getting your email into the user’s inbox - is a discipline in itself. Email clients like Gmail want to stop spam getting into the inbox, so naturally it’s sometimes a dark art - because they don’t want to give the game away. This is increasingly powered by machine learning, so it can be that there literally aren’t any simple rules to follow. That said, here are some starter points:
You may have seen lists of “spammy words” not to include in emails. On the most part, deliverability these days has moved on from the content of your email. It’s about the reputation of your IP address, domain and send tools - and increasingly about how much email clients see users reading your email.
The exception for this is some very old school enterprise type IT systems, which might use systems like Barracuda - which does to some extent scan content and make assumptions based on it. If you’re sending niche B2B emails you might want to dig into this further, otherwise it’s not a major concern.
More than ever, it’s important to not break the rules around deliverability - not just for sender reputation reasons, but also for legal reasons. Things like GDPR and CASL are there to ensure we’re responsible. This isn’t legal advice - but only mail people who have clearly opted in, and you can demonstrate that (you know when and which form they used, if they ask). It might be tempting to mail people who have unsubscribed under the belief that you’re sending vital or servicing information, rather than marketing - this isn’t a decision to take lightly, and if in doubt, it’s time to play it safe.