Metrics Used to Gauge Deliverability<\/h3>\n\n\n\n\n- Delivery rate<\/strong> measures how many emails were accepted by the receiving server, whether they ended up in the inbox or spam.<\/li>\n\n\n\n
- Bounce rate<\/strong> shows emails that could not be delivered at all. Hard bounces are permanent failures such as invalid addresses. Soft bounces are temporary issues like a full mailbox.<\/li>\n\n\n\n
- Spam complaints<\/strong> track how many recipients mark your emails as spam. High complaint volumes directly harm your sender reputation with ISPs and can trigger automatic filtering.<\/li>\n\n\n\n
- Open rates and CTR<\/strong> serve as indirect deliverability signals. Low engagement can indicate messages are landing in spam, though open rates have become unreliable for the reasons covered below.<\/li>\n<\/ul>\n\n\n\n
Why Open Rates Are Unreliable in 2026 (Apple MPP)<\/h3>\n\n\n\n
Traditional open tracking embeds a small invisible image in each email \u2014 when it loads, the open is recorded. Apple’s Mail Privacy Protection (MPP), introduced in September 2021, pre-loads these images on Apple’s servers before the recipient ever opens the message, inflating your reported open rates with phantom reads.<\/p>\n\n\n\n
Apple Mail now accounts for approximately half of all email opens globally. A large share of the open rates you see in Mailgun’s dashboard are pre-loaded proxies, not real reads. Use click-through rate as your primary engagement metric \u2014 it is far harder to inflate artificially and directly reflects whether recipients are acting on your content. For deeper context on improving your engagement signals, see Mastering Global Email Deliverability<\/a>.<\/p>\n\n\n\nChallenges in Measuring Deliverability Accurately<\/h3>\n\n\n\n\n- Lack of visibility:<\/strong> once an email is accepted by the receiving server, you typically cannot tell whether it landed in the inbox or spam without running dedicated placement tests.<\/li>\n\n\n\n
- ISP filtering algorithms:<\/strong> each ISP applies its own rules, which change frequently. The same sending behavior can produce different outcomes across Gmail, Outlook, and Yahoo.<\/li>\n\n\n\n
- Diverse recipient behaviors:<\/strong> different email clients handle messages in ways that affect engagement metrics independently of deliverability.<\/li>\n\n\n\n
- Feedback loop limitations:<\/strong> some ISPs offer feedback loops, but the data is often incomplete and does not always identify the root cause of placement failures.<\/li>\n<\/ul>\n\n\n\n
Common Issues Affecting Deliverability<\/h2>\n\n\n\nPoor List Hygiene<\/h3>\n\n\n\n
Not regularly cleaning your list leads to elevated bounce rates, which signal to ISPs that your list management practices are poor. Sending to unengaged subscribers who have not opened or clicked in months also damages your sender reputation over time. Segment these contacts out and either run a re-engagement campaign or remove them entirely. For practical tools to help, this guide to email list scrubbing<\/a> covers the options.<\/p>\n\n\n\nLow Sender Reputation<\/h3>\n\n\n\n
Consistently sending to invalid addresses signals poor list hygiene to ISPs, lowering your reputation over time. Gmail’s sender guidelines<\/a> require all senders to keep spam complaint rates below 0.10%, with bulk senders never exceeding 0.30%. Frequent complaints cause ISPs to filter your future messages automatically. For a full breakdown of what shapes domain reputation, see From Good to Great: Factors That Affect Domain Reputation<\/a>.<\/p>\n\n\n\nContent That Triggers Spam Filters<\/h3>\n\n\n\n\n- Misleading subject lines:<\/strong> sensational or deceptive subject lines trigger spam filters and reduce recipient trust.<\/li>\n\n\n\n
- Overuse of sales language:<\/strong> phrases like “Buy now!” or “Limited time offer!” are flagged by spam detection algorithms, especially when combined with other risk signals.<\/li>\n\n\n\n
- Poor HTML coding:<\/strong> emails with broken or non-standard HTML are more likely to be filtered, particularly by stricter mailbox providers like Outlook.<\/li>\n<\/ul>\n\n\n\n
Missing or Misconfigured Authentication Records<\/h3>\n\n\n\n
Since February 2024, Google’s email sender requirements make SPF and DKIM mandatory for all senders to Gmail accounts. Bulk senders (5,000 or more messages per day) must also configure DMARC. Gmail began actively enforcing these rules in November 2025, with temporary and permanent rejections for non-compliant senders. The table below shows what each protocol does and who it applies to.<\/p>\n\n\n\n
Why Open Rates Are Unreliable in 2026 (Apple MPP)<\/h3>\n\n\n\n
Traditional open tracking embeds a small invisible image in each email \u2014 when it loads, the open is recorded. Apple’s Mail Privacy Protection (MPP), introduced in September 2021, pre-loads these images on Apple’s servers before the recipient ever opens the message, inflating your reported open rates with phantom reads.<\/p>\n\n\n\n
Apple Mail now accounts for approximately half of all email opens globally. A large share of the open rates you see in Mailgun’s dashboard are pre-loaded proxies, not real reads. Use click-through rate as your primary engagement metric \u2014 it is far harder to inflate artificially and directly reflects whether recipients are acting on your content. For deeper context on improving your engagement signals, see Mastering Global Email Deliverability<\/a>.<\/p>\n\n\n\n Not regularly cleaning your list leads to elevated bounce rates, which signal to ISPs that your list management practices are poor. Sending to unengaged subscribers who have not opened or clicked in months also damages your sender reputation over time. Segment these contacts out and either run a re-engagement campaign or remove them entirely. For practical tools to help, this guide to email list scrubbing<\/a> covers the options.<\/p>\n\n\n\n Consistently sending to invalid addresses signals poor list hygiene to ISPs, lowering your reputation over time. Gmail’s sender guidelines<\/a> require all senders to keep spam complaint rates below 0.10%, with bulk senders never exceeding 0.30%. Frequent complaints cause ISPs to filter your future messages automatically. For a full breakdown of what shapes domain reputation, see From Good to Great: Factors That Affect Domain Reputation<\/a>.<\/p>\n\n\n\n Since February 2024, Google’s email sender requirements make SPF and DKIM mandatory for all senders to Gmail accounts. Bulk senders (5,000 or more messages per day) must also configure DMARC. Gmail began actively enforcing these rules in November 2025, with temporary and permanent rejections for non-compliant senders. The table below shows what each protocol does and who it applies to.<\/p>\n\n\n\nChallenges in Measuring Deliverability Accurately<\/h3>\n\n\n\n
\n
Common Issues Affecting Deliverability<\/h2>\n\n\n\n
Poor List Hygiene<\/h3>\n\n\n\n
Low Sender Reputation<\/h3>\n\n\n\n
Content That Triggers Spam Filters<\/h3>\n\n\n\n
\n
Missing or Misconfigured Authentication Records<\/h3>\n\n\n\n
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