Troubleshooting Email Delivery Delays and Bounce-Backs

A delayed or bounced email is frustrating to diagnose blind — but the bounce message itself almost always contains the actual reason, if you know where to look.

Start with the bounce message

A non-delivery report (NDR) includes a status code and a plain-text reason near the top. Common patterns:

• “550 5.1.1 user unknown” — the recipient address doesn’t exist; check for a typo.
• “550 5.7.1 … blocked” or mentions of Spamhaus/SORBS — your sending IP is on a blocklist (see below).
• “Message delayed” without a hard failure — usually a temporary issue on the recipient’s end; most servers keep retrying for 24-72 hours before giving up.

Checking if your server IP is blocklisted

Use a tool like MXToolbox’s blacklist checker against your server’s IP. If it appears on a list, most blocklists provide a self-service delisting request — our support team can also assist directly since server-level IP reputation sometimes needs a hosting-side fix (e.g. confirming reverse DNS/PTR records are correct).

Delivery delays that aren’t bounces

If mail eventually arrives but slowly, check:

• Whether the recipient’s spam filter is greylisting your server (a legitimate anti-spam technique that deliberately delays first-time senders briefly).
• Whether your domain’s SPF/DKIM/DMARC records are all valid — failing authentication can push a message into slower filtering queues rather than an outright bounce.
• Server load — a very high volume of outgoing mail queued at once can create a backlog; sending in smaller batches avoids this for newsletter-style sends.

When to escalate to support

If the bounce message references a server-side block, blocklist listing, or anything you can’t self-diagnose from the NDR text, open a ticket with the full bounce message attached — it gives our team everything needed to investigate the exact cause immediately.