--- title: "Deliverability" description: "9 posts about Deliverability, summarised, each linking to the original." --- # Deliverability > 9 posts about Deliverability, summarised, each linking to the original. ## Articles ### [Cyber Week Readiness: A Developer's Guide to Scalable Comms](https://yomu.fyi/post/cyber-week-readiness-a-developer-s-guide-to-scalable-comms.md) - Company: [Twilio](https://yomu.fyi/company/twilio.md) - Author: Esther Balta - Published: Oct 13, 2026 - Type: Explainer Cyber Week compresses a year’s worth of system failures into 72 hours, making it the ultimate stress test for communication infrastructure. Developers must optimize APIs with rate limiting, caching, and traffic shaping to absorb sudden spikes without dropping connections. Email delivery requires early IP warmup, SPF/DKIM/DMARC configuration, and limiting promotional sends to recently engaged customers to maintain inbox placement. Voice pipelines need proactive capacity planning for Calls Per Second quotas, fallback URLs, and STIR/SHAKEN attestation to prevent fraud and dropped calls. Monitoring should use a unified dashboard with per-channel metrics, correlation IDs, and pre-written runbooks rehearsed in game days to turn inevitable failures into routine events. ### [Peak Season Deliverability: Why Quality Beats Quantity This Holiday Season](https://yomu.fyi/post/peak-season-deliverability-why-quality-beats-quantity-this-holiday-sea.md) - Company: [Twilio](https://yomu.fyi/company/twilio.md) - Author: Adam Morales - Published: Oct 7, 2026 - Type: Problem & solution During Black Friday and Cyber Monday, email volume spikes, tempting senders to blast all contacts. However, sending to unengaged recipients leads to throttling, blocklists, and spam placement, which can delay time-sensitive holiday offers. Inbox providers use dynamic throughput based on real-time sender reputation; high engagement unlocks maximum sending velocity, while low engagement causes rate-limiting with 4xx deferrals, potentially delivering a 9 AM campaign at 2 AM. Senders with excellent engagement experience up to 100 times faster delivery than those ignoring best practices. Implementing strict sunset policies for contacts inactive 90-180 days, permission-based opt-ins, and automated verification via the SendGrid Email Validation API protects sender reputation and preserves control over campaign timing, avoiding blocklist events that take 24-72 hours to mitigate. ### [Prioritize your messages at scale with the Twilio Traffic Optimization Engine](https://yomu.fyi/post/prioritize-your-messages-at-scale-with-the-twilio-traffic-optimization.md) - Company: [Twilio](https://yomu.fyi/company/twilio.md) - Author: Will McKenzie - Published: Oct 1, 2026 - Type: Announcement Twilio’s Traffic Optimization Engine is a new suite for global messaging deliverability and prioritization, tackling carrier network strain from soaring volumes like the 6.9 billion messages during Cyber Week 2025. The suite automates months of manual planning, working across networks, senders, geographies, and use cases to deliver high-priority messages on time. Real-time Routing uses patented algorithms to optimize traffic across 4,800+ carrier connections every 75 seconds, while Market Throughput manages rate limits at the country and sender-type level to prevent throttling. Traffic Shaping creates dynamic lanes to prioritize one-time passwords over promotional messages, and Multitenancy fairly distributes throughput among ISV teams. Insights provides real-time analytics on deliverability and capacity allocation, and the overall system uses traffic shaping and priority queueing based on carrier, sender, and message type capabilities. ### [Warmup Services Harm Your Deliverability](https://yomu.fyi/post/warmup-services-harm-your-deliverability.md) - Company: [Resend](https://yomu.fyi/company/resend.md) - Author: Phil Nash - Published: Sep 30, 2026 - Type: Explainer Warmup services that simulate email engagement harm deliverability because inbox providers detect the pattern and tighten filtering, causing reputation decline that makes recovery harder. Validity’s new Heatwave blocklist identifies domains warmed through synthetic engagement, using signals from its Intelligence Network to automatically list domains crossing a threshold, and over 1.2 million domains have already been listed. The signal is shared with providers including Comcast, Proofpoint, Spamhaus, and SURBL, potentially damaging domain reputation with these providers. A domain on Heatwave receives a permanent record, and even if the domain stops cold outreach, the warming listing stays, making recovery difficult or impossible. Legitimate senders should verify their domain at lookup.validity.tools, obtain proper consent, and increase volume gradually through real engagement, as the domain warms itself through real engagement and there is no shortcut to trust; warmup activity may also run through agencies without the sender’s knowledge. ### [10+ best practices to improve your email deliverability](https://yomu.fyi/post/10-best-practices-to-improve-your-email-deliverability.md) - Company: [Twilio](https://yomu.fyi/company/twilio.md) - Author: Jesse Sumrak - Published: Sep 9, 2026 - Type: Tutorial Email deliverability determines whether messages reach inboxes rather than merely being accepted by recipient mail servers, and the guide distinguishes delivery rate from inbox placement. It recommends authenticating domains with SPF and DKIM, using appropriate IP allocation and warm-up, applying double opt-in, writing genuine subject lines, offering preference controls, cleaning lists, avoiding purchased addresses and spam traps, sending valuable content, and monitoring metrics. The stated benchmark for good deliverability is at least 95%, while the excerpt cites Twilio SendGrid's 99% API rate and warns that changing providers does not repair domain or IP reputation. Case studies report Shopify at 99.5% overall delivered and 91.3% inbox placement, and Klaviyo at 99.94% average delivery, with the latter also reporting 1.9-second median delivery speed. ### [Holiday email marketing best practices for 2026 (and beyond)](https://yomu.fyi/post/holiday-email-marketing-best-practices-for-2026-and-beyond.md) - Company: [Twilio](https://yomu.fyi/company/twilio.md) - Author: Jesse Sumrak - Published: Sep 8, 2026 - Type: Tutorial Holiday email marketing carries higher volume and stakes during the seasonal shopping period, making planning, deliverability, personalization, and post-holiday retention central concerns. The guide recommends warming sending volume before Black Friday and Cyber Monday, increasing it by only 25–40% at a time to avoid sudden mailbox-provider signals. It also calls for a calendar with one or two lower-volume days each week, list hygiene, segmentation, testing, and journey-driven campaigns that account for shipping and fulfillment constraints. For holiday-season signups, the post-holiday process uses sign-up date or source cohorts, monitors open and click rates, and can reduce frequency, run re-engagement campaigns, or suppress addresses after 30–60 days without engagement. Examples and templates emphasize concise, value-driven content, specific offers, and personalization such as location or past purchases. ### [Welcoming Kiersti Esparza, our new Deliverability Engineer](https://yomu.fyi/post/welcoming-kiersti-esparza-our-new-deliverability-engineer.md) - Company: [Resend](https://yomu.fyi/company/resend.md) - Published: Sep 3, 2026 - Type: Announcement Resend introduces Kiersti Esparza as a Deliverability Engineer on its Trust & Safety Squad, where she will help scale sending infrastructure as the company grows. She brings nearly 20 years in email deliverability, including building teams at Marketo, consulting at Twilio, and deliverability engineering at Klaviyo. Esparza describes the work as a combination of interpreting datasets, understanding email technology, and adapting to evolving authentication in an ongoing contest between bad actors and receivers. She says Resend attracted her with developer-oriented infrastructure, hands-on foundational work, and a culture centered on transparency and partnership, while advising operators to master overlooked infrastructure because its scarcity creates leverage. ### [Welcoming Matt Gilbert, our new Data Analyst](https://yomu.fyi/post/welcoming-matt-gilbert-our-new-data-analyst.md) - Company: [Resend](https://yomu.fyi/company/resend.md) - Published: Jul 9, 2026 - Type: Announcement Resend announces Matt Gilbert as a Data Analyst on its Trust & Safety team. He spent 10 years as a Deliverability Engineer at Mailchimp, handling Delivery-team data and maintaining sending infrastructure that pushed around a billion emails a day, after earlier work on the Mandrill transactional mail service. Gilbert describes email delivery as a multidisciplinary field spanning MTA operations, data engineering, analytics, mail-queue performance, and troubleshooting delivery and inbox-placement problems, which led him to specialize in data engineering and analytics. He says he joined Resend because email remains an open, globally available way for services to communicate, and plans to help build data-driven capabilities and intelligence that enable customers to send better and reach inboxes quickly. ### [Six Steps to Improve Your Sender Reputation](https://yomu.fyi/post/six-steps-to-improve-your-sender-reputation.md) - Company: [Resend](https://yomu.fyi/company/resend.md) - Published: May 20, 2026 - Type: Tutorial Sender reputation is the largely private assessment that inbox service providers such as Gmail, Outlook, and Yahoo make before deciding where to deliver email. It reflects infrastructure compliance, sending history and consistency, list hygiene, mail frequency, and recipient engagement, with providers increasingly emphasizing domain reputation over IP reputation. The guide recommends six recovery steps: audit SPF, DKIM, and DMARC; warm up new domains gradually; identify provider-specific blocks or blocklists; clean and segment lists; make unsubscribing easy; and send valuable, mobile-friendly content. It also advises using double opt-in, monitoring engagement, bounce and spam responses, throttling, and DMARC reports, while tightening policies or requesting provider reviews after fixes. Because providers continually reevaluate senders, reputation management requires repeated observation, testing, and adjustment rather than a one-time repair.