How to diagnose multi-brand shared domain problems?

You've got two brands sending from the same domain, and something's broken. Complaints are up, placement is tanking, or one mailbox provider is suddenly rejecting your mail. The question is: which brand caused it, and how do you fix it without blowing up the other one?

Here's how to work through it systematically.

Step 1: Pull metrics per brand, not just per domain.

If your ESP gives you stream-level or tag-level reporting, use it now. You're looking for which brand's campaigns correlate with the timing of the problem. Compare open rates, click rates, bounce rates, and spam complaint rates for each brand over the past 30 to 90 days. A spike in complaints from Brand B's last promo that lines up exactly with your deliverability drop is your first real clue. Tools like Postmark and SparkPost (now Bird) offer per-stream analytics that make this comparison much easier.

Step 2: Check which provider is affected.

Is the problem showing up at Gmail only? At Outlook? Everywhere? Different mailbox providers weight reputation signals differently. Gmail leans heavily on engagement and complaint rates. Microsoft tends to react more to volume patterns and authentication gaps. If only one provider is affected, that narrows down which signal is triggering it, and which brand's audience lives there.

Step 3: Trace complaint sources.

And your feedback loop data (if you have FBL access from providers like Outlook) will tell you which specific campaigns generated complaints. Match those campaign identifiers back to a brand. If Brand A sends to a well-maintained list of opted-in subscribers and Brand B blasts a cold or stale list, you already know who the culprit is. High complaint rates above 0.08% at Google or 0.3% at Microsoft are enough to drag down the domain's reputation for both brands.

Step 4: Look at bounce patterns.

Now a sudden rise in hard bounces often means one brand is sending to an old or unvalidated list. Check whether bounces cluster around one brand's sends. If Brand B is responsible for 80% of your hard bounces, that's your contamination source. (Worth cleaning that list before sending another thing.)

Step 5: Decide whether to isolate or enforce.

Once you've found the source, you have two real options.

• Isolate: Move the brands onto separate subdomains. Brand A sends from mail.brandA.com, Brand B sends from mail.brandB.com. Each builds its own reputation independently. This is the cleanest long-term fix, especially if the brands serve different audiences with different engagement patterns.
• Enforce shared standards: If separation isn't feasible yet, set non-negotiable sending standards for every brand on the domain. That means authenticated sends with SPF, DKIM, and DMARC in place, regular list hygiene for all brands, complaint rate monitoring with automatic suppression triggers, and agreed volume ramp protocols when sending to a new segment.

One thing to watch for going forward: combined volume. When two brands share a domain, their total send volume is what mailbox providers see. If Brand A sends 100,000 emails a week and Brand B adds another 200,000, that's 300,000 emails all carrying the same domain reputation. A sudden volume spike from either brand can look suspicious on its own, even if the content is fine.

If you're not sure where to start, our free Blocklist Checker can tell you whether your domain has already landed on a major blocklist as a result of the problem. Sometimes that's the fastest way to confirm contamination has already spread.