- Sender reputation, defined
- Sender reputation is the score mailbox providers keep for your sending domain and IP address, and it decides whether your mail reaches the inbox, the spam folder, or nothing at all.
The inputs are bounce rate, spam complaints, spam-trap hits, how consistent your volume is, and above all whether people open and reply. Engagement is the heaviest signal at Gmail.
Reputation is slow to build and quick to lose. A single send to a badly maintained list can undo months, because the complaint and bounce spike arrives all at once.
Verification addresses one input, the bounce rate. It does nothing for complaints or engagement, which is why a clean list alone does not rescue a domain that is already filtered.
How ZapBounce reports it
Reputation is not something a verifier can measure, and we do not report a score for it. Verification addresses one input to it, your bounce rate, and we are explicit that the other inputs are outside what we see.
One reactivation send and what followed
Suppose a retailer with a healthy weekly list of 40,000 decides to add 30,000 old customers who haven't been mailed in two years. The combined send goes out on Monday. About 2,400 of the old addresses hard bounce, which is 8% of that segment, and complaints on the campaign reach 0.4%.
By Wednesday, mail to Gmail starts coming back with a temporary refusal: 421 4.7.28, with text saying the system has detected an unusual rate of unsolicited mail and is rate limiting the sender. Outlook keeps accepting everything, and the open rate there quietly halves, which usually means the junk folder.
Notice who pays. The 40,000 engaged subscribers, who did nothing wrong, now get the weekly email late or in spam. Reputation attaches to the sender, so one bad segment drags down mail to everyone else.
Reading your standing one provider at a time
You already hold most of the evidence. Export a month of send logs and group every metric by the recipient's domain: bounce rate, deferrals, opens, clicks. A problem at one provider shows up as that row drifting away from the others while the account-wide average barely moves.
Read the refusal text too. Providers tell you a surprising amount in 4xx and 5xx replies, including whether the objection is to your IP, your domain, your authentication or your complaint level. That text is the closest thing to a direct statement you'll get.
Then add the provider's own view where one exists. Google Postmaster Tools shows the spam complaint rate Gmail has recorded for your domain, and Microsoft's SNDS reports on traffic from your IPs. Neither will tell you about the other's opinion, and that's the reason to check them separately.
Sender reputation: common questions
How long does it take to repair a damaged reputation?
Weeks of consistent, engaged sending at controlled volume. There is no reset button, and no verifier can shorten it.
Does cleaning my list fix deliverability?
It fixes bounces. If the problem is complaints, poor engagement or authentication, a clean list on its own will not move inbox placement.
Do Gmail and Microsoft score senders the same way?
No. Gmail weighs engagement heavily, Microsoft leans more on complaint rates and its own filtering signals. A good record at one is not portable.