ISPs evaluate four primary signals when deciding whether your email reaches the inbox or gets filtered to spam. Understanding these factors reveals which optimization lever will have the biggest impact on your campaigns.

Sender reputation and dedicated IPs

Your sender reputation acts as a credit score that ISPs reference every time you send an email. This score attaches to your IP address and domain, making infrastructure choices critical for deliverability.

Shared IP pools create risk because other senders' behavior affects your deliverability. Mailtrap's deliverability research explains that one bad actor on a shared pool can trigger blacklisting that affects every sender using that IP range.

Dedicated IPs work differently. Since the IP belongs to just one sender, everything you do shapes its reputation. For agencies managing multiple client campaigns, this isolation prevents cross-contamination when one client's campaign underperforms.

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Our dedicated IP vs shared IP guide details how Google Workspace places cold email campaigns on shared pools where one spammer can tank deliverability for everyone, while dedicated IPs isolate your reputation so your behavior alone determines inbox placement.

SPF, DKIM, and DMARC authentication

These three protocols prove to receiving servers that your emails are legitimate and haven't been spoofed by bad actors.

SPF (Sender Policy Framework) enables domain owners to publish DNS records listing which email servers can send on their behalf. Cloudflare's email security documentation explains that ISPs check this list to verify the sending server is authorized.

DKIM (DomainKeys Identified Mail) uses cryptographic signatures to verify email integrity. According to Valimail's protocol guide, your email system signs messages with a private key, and receiving servers verify the signature using your public DNS record.

DMARC (Domain-based Message Authentication, Reporting & Conformance) tells receiving servers what action to take when SPF or DKIM checks fail (monitor, quarantine, or reject). Klaviyo's deliverability research found that proper implementation of all three protocols boosts deliverability rates by 5-10%.

List hygiene and bounce rates

Hard bounces signal to ISPs that you're sending to invalid addresses, damaging sender reputation and triggering spam filtering.

Bounce rates above 2% trigger reputation problems, and going from 1.8% to 2.3% isn't a small increase. It's the difference between primary inbox and spam folder. Elite campaigns maintain bounce rates under 1%, while rates above 3% put your sender reputation at critical risk.

Email verification prevents hard bounces by ensuring you don't send to invalid addresses or spam traps. Too many hard bounces will negatively affect your reputation with mailbox providers, making verification a critical step before launching campaigns.

ISP filtering and spam complaints

Spam complaints directly signal to ISPs that recipients don't want your emails. Even a small percentage of complaints can devastate deliverability across your entire sending domain.

Industry benchmarks show you need to keep spam complaints far below 0.3% to protect inbox placement. Exceeding this threshold triggers aggressive filtering that can take weeks to recover from.

ISPs also evaluate engagement signals like opens, clicks, and replies. Cold email reply rates vary widely, but elite senders hitting 10-15% see significantly better inbox placement because positive engagement signals tell ISPs your emails are wanted. Watch the booking 6 calls per day video to see how one user achieves high engagement rates.