Understanding sender reputation

Sender reputation is a score — or a set of scores — that internet service providers (ISPs) and receiving mail servers use to decide whether your emails deserve to reach the inbox. It is not a single number you can look up anywhere. It is the accumulated signal of how your emails have behaved over time, as seen by receiving servers like Gmail, Outlook, and others.

Understanding sender reputation matters most when you are new to Flexmail, when you are migrating from another platform, or when your deliverability has unexpectedly declined. This article explains what it is, what affects it, and what you can do about it.

How reputation is built

Reputation is earned through consistent, expected sending behaviour over time. ISPs track patterns, not single events. What they observe includes:

• Bounce rate: how many of your emails come back as undeliverable. High bounce rates signal that your list is not maintained.
• Spam complaint rate: how often recipients mark your emails as spam. Gmail recommends keeping this below 0.1%. Above 0.3% causes serious deliverability problems.
• Engagement: whether recipients open and click. High engagement signals that recipients want your emails. Low engagement over time suggests they do not, even if they have not unsubscribed.
• Authentication: whether your SPF, DKIM, and DMARC records are correctly configured. Unauthenticated emails from your domain damage the domain's reputation.
• Sending consistency: sudden large spikes in volume look suspicious to ISPs, especially from a new or inactive sending domain or IP.

How reputation is damaged

Reputation damage accumulates faster than it is repaired. The most common causes:

Sending to stale or purchased lists

Old contacts who have not heard from you in years, and purchased lists who never opted in, generate high bounce rates and spam complaints. ISPs treat the resulting patterns as a signal that you are not a trustworthy sender. The damage affects all your future sends, not just the campaign that caused the problem.

High bounce rates

A consistent hard bounce rate above 2% is a warning sign. Flexmail automatically moves hard-bounced addresses to your blacklist, which helps, but if you are importing large batches of unverified addresses, the damage happens before Flexmail can catch it.

Spam complaints

Even a small number of spam complaints relative to sends has an outsized effect. Each complaint is a direct negative signal to the ISP from one of their own users. Gmail and Outlook both weigh complaint rates heavily in inbox placement decisions.

Sending to unengaged contacts

Contacts who never open your emails are not neutral. Over time, ISPs interpret mass sending to consistently non-opening recipients as a sign that your list was not built with genuine interest. This is one reason why re-engagement and list pruning are deliverability strategies, not just list management strategies.

IP warming for new accounts and migrations

When you start sending from a new IP address — which happens when you create a new Flexmail account or switch from another platform — that IP has no sending history. ISPs treat unknown IPs cautiously. Sending large volumes immediately from a cold IP is one of the fastest ways to damage your reputation before you have even built it.

IP warming means gradually increasing your sending volume from a new IP so that ISPs can build a positive reputation signal over time. The principle is: start small, with your most engaged contacts, then scale up as your delivery rates confirm inbox placement is good.

A typical warming approach over the first four to six weeks:

• Week 1: send only to your most recent, most engaged contacts — people who have opened or clicked in the last 30 to 60 days. Keep volume low (a few hundred to a few thousand, depending on your list size).
• Weeks 2 and 3: if delivery rates and complaint rates look healthy, gradually expand to contacts who have engaged in the last 90 to 180 days.
• Weeks 4 to 6: continue expanding to your broader list, watching bounce and complaint signals at each step.

Domain reputation vs. IP reputation

ISPs track reputation at two levels: the sending IP address and the sending domain (the part of your email address after the @). Flexmail manages the IP-level reputation on your behalf through its shared infrastructure. Your domain reputation is your responsibility.

Domain reputation is what you build or damage through your sending behaviour. It is also what persists when you switch email platforms: your domain's history follows you. This means that if your domain has been used to send to purchased lists or generate spam complaints on a previous platform, that history will affect your deliverability on Flexmail as well.

Correct authentication (SPF, DKIM, DMARC) is the foundation of domain reputation. Without it, receiving servers cannot verify that your emails genuinely come from your domain, and inbox placement suffers regardless of your sending behaviour.

How to monitor your reputation signals in Flexmail

Flexmail does not show a single "reputation score" — no such universally agreed-upon number exists. What you can monitor:

• Bounce rate per campaign: visible in every campaign report under Deliveries. A rising trend across campaigns is a warning sign.
• Spam complaint rate: visible in campaign reports. Contacts who mark your email as spam are automatically blacklisted. Review the Blacklisted tab in Contacts > All contacts after each campaign.
• Open and click rates: declining engagement over time can precede deliverability problems. Use click rate and click-to-open rate (CTOR) as your primary engagement signals, as open rate became less reliable after Apple's Mail Privacy Protection in 2021.
• Authentication status: go to Settings > Add or remove senders and check that SPF, DKIM, and DMARC indicators are green. DNS records can stop working if your domain settings are changed by someone on your IT team.

For external monitoring, tools like mail-tester.com and Google Postmaster Tools (for Gmail delivery specifically) provide additional data points. Google Postmaster Tools shows domain reputation and spam rate as seen by Gmail directly.

What to do if your reputation is damaged

Reputation repair is slower than reputation damage. There is no quick fix, but the steps below consistently work over a period of weeks to months:

1. Stop sending to your full list immediately. Sending more campaigns with an already damaged reputation accelerates the problem.
2. Identify and remove the cause: stale addresses, purchased contacts, contacts without genuine consent. Segment your list and keep only those who have actively engaged recently.
3. Verify and fix authentication. Confirm SPF, DKIM, and DMARC are all green in your account settings.
4. Re-warm your sending. Start again with a small, highly engaged segment and rebuild volume gradually, exactly as you would with a new account.
5. Monitor at each step. Check bounce and complaint rates after each campaign before expanding.

Common mistakes to avoid

Sending a large campaign immediately after migrating

Sending to your full list on day one of a new Flexmail account is the most common cause of early deliverability problems for new customers. Warm up first.

Not importing unsubscribers before the first send

This cannot be overstated. Contacts who unsubscribed on your previous platform expect not to receive emails from you. Sending to them generates spam complaints that damage your reputation from the very first campaign. Import your unsubscribers to the Flexmail blacklist before you send anything.

Conflating list size with deliverability

A large list does not automatically mean good deliverability. A clean list of 5,000 highly engaged contacts will consistently outperform a bloated list of 50,000 contacts with poor engagement. Focus on list quality over list size.

Reactivating old contacts without a re-engagement campaign

Contacts who have not heard from you in 12 months or more should not receive your regular campaigns immediately. They are much more likely to mark the email as spam because they no longer recognise you. Run a re-engagement campaign first.

Next steps

• See 'Sender validation and email authentication' for how to set up SPF, DKIM, and DMARC for your sending domain.
• See 'The blacklist' for how to import unsubscribers from another platform before your first send.
• See 'My emails are landing in spam' if you are experiencing active deliverability problems.