What are the most common causes of sudden deliverability drops?

Sudden deliverability drops, where yesterday everything was fine and today nothing's landing, almost always have one of a handful of causes. Here's how to identify which one you're dealing with.

Blocklist listing. Your sending IP or domain appeared on a major blocklist. This causes immediate, widespread rejection at providers that query that list. Check first. It's the fastest thing to rule in or out. Our free blocklist checker covers most major lists.

Authentication failure. Your SPF, DKIM, or DMARC broke. This happens when someone changes DNS settings without realizing the effects, when a DKIM key gets rotated, or when an ESP migration doesn't transfer auth records properly. Check your DNS. Did anyone make changes recently?

Spam complaint spike. A single send to the wrong segment, an old list, a purchased list, an unengaged segment, can generate enough complaints to trigger filtering. Look at your complaint rate in your ESP dashboard (or Google Postmaster Tools if you're primarily Gmail-affected). If it spiked above 0.1%, that's your problem.

List quality event. Sending to a batch of addresses that haven't engaged in years, a purchased list, or addresses that include spam traps. Hard bounces above 2% are a strong signal.

Volume spike. Suddenly sending 10x your normal volume looks suspicious to ISPs. Reputation is built at a volume level. Jumping beyond it gets you throttled.

ESP or IP change. If you recently switched ESPs, moved to a new sending IP, or changed your from domain, that's likely the cause. New infrastructure has no reputation yet.

Start with blocklist check and authentication verification. Both take under five minutes and cover the most common causes. If those are clean, look at the complaint rate in the days leading up to the drop.