Email deliverability is the ability of your emails to reach the inbox rather than the spam folder or nowhere at all. It's decided by three things together.
1. Setup
SPF, DKIM and DMARC on every sending domain, an MX record so replies arrive, and real mailboxes from a reputable provider such as Google Workspace.
2. Reputation
How providers rate your domain and sending servers, built from bounces, spam complaints, engagement and blocklists. New domains have none, which is why they need warm-up.
3. Sending habits and content
Modest volume per mailbox (around 30 cold emails a day), a verified list, the right audience, and short, personal emails with few links and no attachments.
How to tell if it's slipping
Falling reply rates, rising bounces, warm-up emails landing in spam or a domain on a blocklist. Run an inbox placement test and check the domain with the free domain health checker.