Email lands in the spam folder for a small number of reasons, and almost all of them are fixable. The good news is that the fixes are the same whether you are sending transactional email, marketing, or cold outreach: get the authentication right, get off the blocklists, and send from a reputation that is not broken. This is the order to work through them, because each step depends on the one before it.
Most "my email is in spam" problems are an authentication problem in disguise. The receiving mailbox checks three records, and if any of them is missing or wrong, the email is treated as untrusted. Check that SPF lists every server that sends for you and ends in -all. Check that DKIM is signing on the domain in your From header. Check that DMARC is present. If you are not sure which of the three is broken, the SPF, DKIM and DMARC checker runs all three in one pass and tells you exactly which record is the problem. This is the highest-leverage fix: a single missing DKIM selector will keep you in spam no matter how clean everything else is.
Even with perfect authentication, a single blocklist hit will sink you. Check the sending IP and the domain against the major lists. The IP blacklist checker and domain blacklist checker show which lists you are on and, more importantly, the delisting process for each one. For the big ones like Spamhaus, the Spamhaus delisting guide walks through the exact request. The key point: delisting only works if the underlying cause (a compromised mailbox, an open relay, a burst of bad traffic) is fixed first, or you will just get listed again within days.
Authentication and blocklists are the floor. What keeps you out of spam long-term is sending behavior. Keep volume gradual, especially if you have been quiet or are sending from a new domain: ramp up over days, not hours. Keep your bounce rate low by validating addresses before you send. Make sure your content matches your sending domain (a new domain suddenly sending heavy promotional email is a classic trigger). If you are doing cold outreach, this is where warm up matters: a fresh domain needs a reputation built before it can send at volume.
The reason spam-folder problems recur is that nobody is watching. A record breaks, an IP gets listed, a mailbox gets compromised, and the next campaign dies quietly. The fix is continuous monitoring: check the domain daily, get alerted the moment a record changes or a blocklist hit appears, and fix it in minutes instead of discovering it from a drop in deliverability. That is what Inboxproof Pro does, and the Agency tier does the same for up to ten client domains with white-label reports.
Work the steps in this order: authentication first, then blocklists, then sending behavior, then monitoring. Skipping ahead is why people "fix" the spam problem and have it back a week later. Start by running the free deliverability checker or the email spam checker on your sending domain. Both score all seven checks and point at the exact record to fix first. Once the floor is solid, the monitoring keeps you out for good.