You can send a thousand emails, but if they land in spam folders, your marketing is worthless. Email deliverability is the rate at which your emails actually reach the inbox. If subscribers tell you they never received your newsletter, it's likely sitting in their spam folder.
To maintain high deliverability, use a reputable email platform, avoid spammy words in subject lines, keep your email list clean by removing inactive subscribers, and properly configure DKIM, SPF, and DMARC authentication. These technical settings prove to email servers that you're a legitimate sender. Also avoid excessive links, images, and suspicious formatting that triggers spam filters.