How do you customize Preheader / inbox preview text intentional?

Set `requiredPreheaderContent` when a campaign family must always carry a specific phrase in the preview — a legally required qualifier, a programme name, an offer identifier. Extend `prohibitedPreviewPatterns` when your templates have their own boilerplate first line that the defaults do not know, such as a mandatory disclaimer or a mirror-page label. Adjust `previewWindowChars` when you know your audience's dominant client and want the front-loading window to match what it really shows.

Keep the defaults when your concern is simply that preview text exists and is not boilerplate, which is what most senders need. Leave `requiredPreheaderContent` empty unless a phrase is genuinely mandatory, because demanding fixed wording in the most valuable line in the inbox is a heavy constraint on copy. Keep both numbers if you have a mixed audience, since 130 and 90 are chosen to be reasonable across clients rather than tuned to one.

Requiring fixed content trades open-rate flexibility for consistency, and every required phrase consumes part of a window that is already short. Extending the prohibited list risks failing legitimate copy — a campaign that genuinely offers an online version early in the body will trip a pattern written to catch boilerplate. Tightening `previewWindowChars` produces more failures on perfectly deliverable preheaders, which is useful only if your team accepts front-loading as a rule rather than a suggestion.

The Standard configuration is written from observed client behaviour and from the boilerplate that real templates actually leak, which is why the required-content field is empty and the prohibited list is not. Custom values are campaign or brand policy and belong with whoever owns campaign standards; a required phrase in particular should be traceable to the obligation or programme that demands it. Review the numbers when your audience's client mix shifts, since both are claims about where a reader stops reading.

A financial services sender sets `requiredPreheaderContent` to the qualifier its compliance team requires in the preview line, so a campaign that omits it fails before sending. A team whose templates open with a mandatory legal disclaimer adds that phrasing to `prohibitedPreviewPatterns`, catching sends where the disclaimer became the preview text. A mobile-dominant consumer brand lowers `previewWindowChars` to `60` to force sharper front-loading, accepting more findings in exchange for preview lines that are read in full on a phone.