Deliverability problems don't announce themselves — they usually surface as a sudden unexplained drop in opens, a spike in bounces, or a complaint from a customer who didn't receive a critical email. This week we walk through a systematic diagnostic process.
This Week’s Lesson
Step 1: Determine scope. Is the problem affecting all recipients or a specific domain (e.g., only Gmail users affected, or only @company.com recipients)? Domain-specific problems point to blocklists or recipient-side filtering. Broad problems suggest sending infrastructure issues.
Step 2: Check authentication. Go to Google Postmaster Tools and check your SPF, DKIM, and DMARC pass rates. Any significant drop below 99% indicates an authentication failure. Check if any new sending tools were added without updating your SPF record.
Step 3: Check blacklists. Run your sending domain and IP through mxtoolbox.com/blacklists.aspx and multirbl.valli.org. A new blacklist listing often explains sudden delivery failures.
Step 4: Check complaint rate. In Google Postmaster Tools, look at your spam rate over the last 14 days. A spike above 0.1% indicates recipient complaints — likely from a recent campaign to a cold or unengaged segment.
Step 5: Check bounce rate in your ESP. A sudden bounce rate increase above 3% usually means you sent to a segment with poor list hygiene — old addresses, imported contacts, or a segment that hasn't been emailed recently.
Step 6: Review recent changes. Any new email campaigns, new sending tools, DNS changes, or IP changes in the last 30 days? Deliverability problems are almost always traceable to a change that introduced a new variable.
Step 7: Run seed tests. Send to seed accounts (test inboxes at Gmail, Outlook, Yahoo) and check which inbox folder the email lands in. If it's in spam, that's your confirmation — now review the content and headers for spam triggers.