February 2023 Summaries

SMTP is the protocol that sends email from clients to outgoing servers and relays it between mail servers, with different ports commonly used for submission and relaying. Port 25, the original SMTP port, is now primarily intended for server-to-server relaying because many providers block it for message submission due to historical spam abuse. Port 587 is the officially recognized and broadly supported default for secure email submission using STARTTLS, while port 2525 is a reliable TLS-capable fallback when 587 is unavailable, despite lacking formal standardization. Port 465 remains in use by some legacy services for SSL-based SMTP but is generally discouraged unless required, and port 1025 may serve as a last-resort submission alternative. Major providers typically use port 587, although Gmail and Yahoo also support 465 for SSL, and deliverability depends not only on port selection but also on factors such as infrastructure, email content, complaint rates, and performance monitoring.

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Email open rate measures the percentage of recipients who open a campaign and serves as an early indicator of email performance, typically calculated using unique opens divided by total recipients or, more accurately, delivered recipients after excluding bounces. Average rates across campaigns can be calculated by averaging individual campaign rates, while click-to-open rate measures the share of openers who also click a link and helps assess content relevance. Opens are usually tracked through invisible pixels, though blocked images and privacy settings can reduce accuracy, while trackable links and read receipts have their own limitations. Performance varies by email type, industry, timing, region, device, list quality, and content, with transactional emails generally receiving more opens than marketing messages. Improving open rates involves using recognizable personal sender names, brand identifiers such as BIMI, concise and relevant personalized subject lines, audience segmentation, and avoiding spam-like language or excessive formatting. The post also recommends monitoring deliverability, bounce and spam rates, and testing authentication, SMTP routing, template rendering, spam scores, and email content before sending campaigns.