If you're having deliverability issues, I can tell you from my experience that you aren't really going to be able to get back to the inbox if any of the following types of subscriber list sources are in play:
- Co-registration or "lead generation." Every smart marketer knows that lead generation isn't inherently evil. But there's a common class of "leadgen," "co-reg" or "affiliate" marketers out there that basically just share any submitted email address with ten, twenty, fifty or even more marketers, all of whom are then competing for attention in that same subscriber inbox. Add yourself to that mix, and your mail is pretty much guaranteed to go to the spam folder, not the inbox. The only fix is to get out of this game. You can't win. Payday loans, insurance quotes, free ipads, whatever, it just doesn't work. Each subscriber's inbox is basically ruined, and nobody reads any of the mail, so low engagement and fingerprinting result in broad spam foldering. (And a lot of payday loan or leadgen senders seem to have ethical challenges beyond email address collection.)
- Email append. Email append, to put it bluntly, is a bad practice. If a subscriber didn't give you their email address, they're not expecting emails from you. You can't just go look up their email address and add it to your list, even though vendors claim that this is a fine practice. What actually happens is you match some of the the wrong people. Even some of the right people will be upset about it. Spam complaints will spike. And spamtrap hits will spike -- because the append vendor's data is not a "triple verified" as they claimed it to be. (Caveat: In theory, email append can be done totally opt-in, with confirmation and everything. In practice, append vendors hate it and often offer it just to make their website look more respectable.) More on why email append is bad can be found here.
- Purchased lists. Unless people knew, at the point of handing over their email address, that they're going to receive mail from you, they're going to find your mail unexpected and it's going to result in spiking spam complaint rates. And spamtrap hits are likely to go up, as well, because list vendors have little recourse to weed out bad addresses -- it makes their lists seem smaller and less impressive. More on why purchasing lists is a bad idea can be found here.
- Very old lists. If you haven't emailed your subscribers in years, that list is dead. Dead, I say. Why? Because email addresses churn. They get retired and repurposed. They get turned into spamtraps. To avoid spamtraps, you start with opt-in permission up front, and then you also have to be careful to watch for and suppress addresses that bounce. A common practice for spamtrap address creation involves retiring an address or domain for eighteen months, then re-enabling it as a "trap," feeding right into a spam filter. If your list sits inactive for four years, you've long missed that eighteen month window for some of those addresses. Spamtrap hits will spike, and you'll get blocked or blacklisted. More on how sending to very old lists will kill your deliverability can be found here. You really should engage with your subscriber base periodically, so you can give your email platform a chance to suppress bouncing and invalid addresses for you automatically, before they get recycled into spamtrap addresses.
There are definitely other problems that can also cause deliverability issues. But if you're mailing to subscriber data that matches any of these practices, the only way you're going to see future deliverability success is to wind down that practice. Retire that old list. Don't buy lists. Don't engage in email append. And don't believe that vendor that tells you otherwise.