Subject lines and preheaders
The subject line and preheader work as a pair. Many phone inboxes show only the first few words of each, so put the point first, then use the preheader to add to it rather than repeat it. If no preheader is set, email clients pull the first text in the message, often 'View in browser' or an image's alt text. Generate variants that differ in approach (benefit, news, specificity, curiosity), not in adjectives. Under CAN-SPAM a subject line must not mislead about the content of the message, so fake 'Re:' and 'Your order' prefixes are out wherever US law applies, and they erode trust everywhere else.
Personalisation that does not break
Personalisation fails in public. A missing first name produces 'Hi ,' and a wrong one is worse than none, so every merge field needs a fallback and every dynamic block a default. Personalise on what the recipient would expect the brand to know, such as what they bought, browsed or chose, not on inferences that feel invasive. Under the GDPR, data used for personalisation needs a lawful basis and should be limited to what is necessary. Review generated copy per segment rather than per email, and check the targeting logic as carefully as the words.
Spam filters care most about the sender
Individual words matter less than they used to. Since February 2024, Gmail and Yahoo have required bulk senders (Gmail counts around 5,000 or more messages a day to its users) to authenticate with SPF, DKIM and DMARC, offer one-click unsubscribe and keep reported spam below 0.3%, and both expect unsubscribes to be honoured within two days. Microsoft added similar requirements for Outlook.com in 2025. Copy still counts at the margins: misleading subject lines, all-capital text, image-only emails and link shorteners look like spam to filters and to people. Consistent sending, clean lists and relevant content do more than any list of forbidden words.
Consent law and accessible HTML
In the EU, the ePrivacy Directive (2002/58/EC) requires prior consent for marketing email to individuals, with a soft opt-in for existing customers buying similar products, and the GDPR sets the standard for valid consent; the UK follows the same pattern under PECR. The US CAN-SPAM Act allows email without prior consent but requires accurate headers, a physical postal address, a working opt-out honoured within 10 business days and identification as an advertisement. Accessible HTML reaches more readers: real text rather than images of text, alt text, logical reading order, sufficient contrast and a language attribute. Gmail also clips messages over 102 KB.
Updated 25 September 2026