Sender reputation is the level of trust that mailbox providers such as Gmail, Outlook and Yahoo place in you as an email sender, based on how recipients and their systems have reacted to your past messages. It is the biggest single factor in whether your emails land in the inbox, the spam folder or nowhere at all.
How sender reputation works
Providers track reputation against two identifiers. IP reputation belongs to the server address that sends your mail; on a mainstream email platform you usually share that address with the platform’s other customers. Domain reputation follows your own sending domain wherever you send from. For most small and medium-sized UK businesses domain reputation matters more, because it moves with you when you change platform.
The signals that build or damage it include:
- your spam complaint rate, meaning how often people mark your email as spam;
- bounces, especially repeated sends to addresses that do not exist;
- hits on spam traps;
- engagement, such as replies, clicks, rescuing your email from spam, or deleting it unread;
- whether SPF, DKIM and DMARC authentication pass and align with your from address;
- how steady your volume is, since a sudden spike from a quiet domain looks like a hijacked account.
No provider publishes its formula, and each keeps its own view of you. You can be in good standing with Outlook and struggling at Gmail at the same time.
Why it matters
A poor reputation is costly and slow to repair. Your emails quietly go to spam, so the list looks healthy in your platform while sales from email fall away. Because domain reputation can also cover ordinary business mail on the same domain, invoices and quotes sent by your accounts team may start going astray too.
The large providers have become stricter. Google’s email sender guidelines ask high-volume senders to authenticate their mail, support one-click unsubscribe and keep the spam rate shown in Google Postmaster Tools below 0.1%, never reaching 0.3%. Those are Google’s own published thresholds; check the current page, as they are revised from time to time.
Common mistakes
- Importing an old or bought list. Stale addresses produce bounces, complaints and spam-trap hits all at once. In the UK, emailing a bought list of individuals is also very unlikely to meet PECR’s consent rules.
- Going from silence to a big send. A dormant domain that suddenly mails tens of thousands of people looks suspicious. Increase volume gradually, as described under domain warming.
- Mailing inactive subscribers indefinitely. People who never engage drag down your signals, and long-abandoned addresses are where recycled traps hide.
- Making unsubscribing awkward. If leaving takes effort, people use the spam button instead.
- Blaming the platform. Moving to a new email tool does not reset domain reputation. The same habits produce the same result.
How to act on it
Start by measuring. Verify your domain in Google Postmaster Tools, which at the time of writing (October 2026) shows your Gmail spam rate and whether you meet Google’s sender requirements, and read your platform’s bounce and complaint figures for every campaign. Confirm that SPF, DKIM and DMARC pass for your sending domain.
Then protect it through habits: only email people who asked to hear from you, remove hard bounces immediately, put a sunset policy in place for long-inactive contacts, and keep your sending volume steady. If your reputation is already damaged, send only to your most engaged subscribers for a few weeks and widen the audience gradually as the figures recover.
Reputation problems usually trace back to how a list was built, which is why I look at them as part of a digital marketing strategy review rather than as a purely technical fix.