What Is Sender Reputation and How to Check Yours

How Sender Reputation Works

Every ISP maintains its own reputation system. When your server connects to deliver an email, the ISP looks up the reputation of your sending IP and domain. If your reputation is good, the email is accepted and delivered to the inbox. If your reputation is moderate, the email may be accepted but placed in spam. If your reputation is poor, the email may be rejected outright.

There are two types of reputation that matter:

IP Reputation

This is the reputation of the IP address your emails are sent from. If you use a dedicated IP from your email provider, you own and control this reputation. If you are on shared IPs, you share the reputation with other senders on those IPs, which means their behavior affects your deliverability. IP reputation is particularly important for Microsoft Outlook, which weights IP reputation heavily in its filtering decisions.

Domain Reputation

This is the reputation of your sending domain (the domain in your From address). Domain reputation follows you even if you change email providers or IP addresses. Gmail weights domain reputation more heavily than IP reputation, meaning you cannot escape a bad domain reputation by switching providers. Domain reputation has become increasingly important as ISPs recognize that spammers frequently change IPs but rarely change domains.

What Factors Affect Your Reputation

• Bounce rate: Hard bounces above 2% signal a poorly maintained list. ISPs see this as a sign that you are sending to addresses you should not have.
• Spam complaint rate: Gmail's threshold is 0.1% (1 in 1,000). Above 0.3% is critical. This is the single most damaging factor for reputation.
• Spam trap hits: Sending to spam trap addresses (old addresses repurposed by ISPs to catch bad senders) is extremely damaging because it proves you are not managing your list.
• Engagement: ISPs track whether recipients open, click, reply to, or forward your emails. High engagement tells the ISP your email is wanted. Low engagement suggests it is not.
• Authentication: Consistent SPF, DKIM, and DMARC passes build trust. Failures raise suspicion.
• Sending consistency: Regular, predictable sending volumes are viewed more favorably than erratic spikes and gaps.
• Blocklist presence: Being listed on major blocklists (Spamhaus, Barracuda, SORBS) severely damages reputation across all ISPs.

How to Check Your Sender Reputation

Google Postmaster Tools (Free)

Google Postmaster Tools is the most important monitoring tool for email senders. It shows your domain reputation with Gmail (rated as High, Medium, Low, or Bad), your IP reputation, authentication rates (SPF/DKIM/DMARC pass percentages), spam rate, and delivery errors. To use it, verify your domain by adding a DNS record and wait 24-48 hours for data to appear. You need at least a few hundred daily sends to Gmail for data to show.

Microsoft SNDS (Free)

Microsoft Smart Network Data Services shows your IP reputation with Outlook, Hotmail, and other Microsoft email. It shows complaint rates, spam trap hits, and a color-coded reputation status (green/yellow/red) for each IP address. Sign up and add your IP ranges to start monitoring.

Sender Score by Validity (Free)

Sender Score provides a 0-100 reputation score for your sending IP. Scores above 80 are good, 70-80 are moderate, and below 70 indicate problems. The score is based on data from ISPs and email providers in the Return Path network. Check your score at senderscore.org.

MXToolbox Blacklist Check (Free)

MXToolbox checks your IP and domain against dozens of major blocklists simultaneously. If you appear on any blocklist, it identifies which one and provides a link to the blocklist's delisting process. Run this check regularly or set up automated monitoring.

Talos Intelligence by Cisco (Free)

Talos provides reputation data used by many email gateways and corporate email filters. It rates your IP and domain as Good, Neutral, or Poor. Corporate email filters from Cisco (used by many businesses) rely on this data.

How to Improve a Damaged Reputation

Immediate Actions

• Remove all hard bounces and addresses that have complained from your list.
• Fix any authentication issues (SPF, DKIM, DMARC failures).
• Check blocklists and submit delisting requests for any you appear on.
• Reduce sending volume temporarily to only your most engaged segment.

Ongoing Recovery (2-4 Weeks)

• Send only to subscribers who have opened or clicked in the last 30 days.
• Keep volumes low and consistent (no spikes).
• Monitor complaint rates and bounce rates on every send.
• Gradually increase volume as reputation metrics improve in Postmaster Tools and SNDS.

Long-Term Maintenance

• Process bounces and complaints automatically after every campaign.
• Remove subscribers who have not engaged in 90+ days.
• Use double opt-in for new subscribers.
• Monitor reputation tools weekly.
• Maintain consistent sending schedules.