A deal is using a date you no longer believe. Sales will hear that as "engineering failed" unless you give them something they can say instead. The draft that feels honest is a tour of the board. The draft they can use is a new promise and a retired one.
This is the internal version of telling a client about a scope change. They still have a customer. You still owe a number they can repeat.
Lead with what they may say
"Do not promise the shared inbox on the 12th. You can still promise search and the export. Shared inbox is the 26th, and only if the vendor sandbox matches production — that last part is an assumption, not a commitment."
Two sentences they can paste. That is the whole job of the first paragraph. Everything else is for you.
If you cannot offer a date, offer the hour when you will. An honest deadline on information is still a deadline. "We will know by Tuesday 15:00 UTC" is a sentence they can take to a buyer. "We are looking at it" is how they invent a Friday.
Give them a decision, not a status
You may need them to move a demo, drop a line from a slide, or accept a cut so the original date holds for something else. Close the list. Name your recommendation.
They will not read a ticket dump long enough to find the question. If the note is only FYI, say so in the first line and do not ask for a reply. Mixed notes produce mixed replies, usually in a thread you then have to untangle during their call.
Do not chase them with the same slip
One note. A default: "If I have not heard by 16:00 UTC I will assume you can move the demo and I will stop work on the inbox for this train." Following up only if the facts changed.
A long technical autopsy belongs in the engineering record, not in their inbox. Three factual lines is enough for them not to invent a story: what broke, what you ruled out, what is left.
Send it before they repeat the old date
You approve the wording. Their assistant, if they have one, should receive a briefing, not a screenshot of Jira. No attachments. Link the ticket if you must.
If they have a RelayLink account, you need an accepted contact. If they do not, they get email and a page. You do not need to be on the same deal channel.
Use the names they use with customers. A nickname you have for an account does not belong in a note they might forward.
If the slip is already obvious in a session, connect your assistant and have it write the two sentences they can say out loud.