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.
Source: [Dev.to](https://dev.to/bucabay/email-to-slack-threading-block-kit-limits-and-the-duplicate-post-trap-1hl0)