Email to Slack: threading, Block Kit limits, and the duplicate-post trap

Start from the mismatch, because every bug in this integration comes out of it. Email hands you a MIME tree, an SMTP envelope, and a Message-ID chain that defines the conversation. Slack hands you a channel, a message of at most 50 blocks, and a ts that defines the conversation. The whole job is mapping one onto the other without dropping information — the reply chain, the authentication verdicts, the attachments — on the floor. Here's the whole inbound half, as a Cloudflare Worker. It runs as pasted with one KV namespace bound as SEEN and one dependency ( npm install mailkite ): // worker.js — inbound email → Slack. wrangler secret put SLACK_BOT_TOKEN / MAILKITE_WEBHOOK_SECRET import { MailKite } from " mailkite " ; const clamp = ( s , n ) => ( s . length > n ? s . slice ( 0 , n - 1 ) + " … " : s ); function blocksFor ( email ) { const subject = email . subject || " (no subject) " ; const trusted = email . auth . dmarc === " pass " ; const sender = trusted && email . from . name ? ` ${ email . from . name } < ${ email . from . address } >` : email . from . address ; return [ { type : " header " , text : { type : " plain_text " , text : clamp ( `📧 ${ subject } ` , 150 ) } }, { type : " section " , fields : [ { type : " mrkdwn " , text : `*From:*\n ${ clamp ( sender , 2000 )}${ trusted ? "" : " ⚠️ " } ` }, { type : " mrkdwn " , text : `*To:*\n ${ email . to [ 0 ]. address } ` }, ] }, { type : " section " , text : { type : " mrkdwn " , text : clamp ( email . text || " _no text part_ " , 3000 ) } }, { type : " context " , elements : [ { type : " mrkdwn " , text : `spf \` ${ email . auth . spf ?? " unknown " } \` · dkim \` ${ email . auth . dkim ?? " unknown " } \` · dmarc \` ${ email . auth . dmarc ?? " unknown " } \` ` }, ] }, ]; } export default { async fetch ( req , env ) { const raw = await req . text (); const sig = req . headers . get ( " x-mailkite-signature " ); // HMAC recompute, constant-time compare, ±5-minute replay window: one call if ( ! MailKite . verify