Asking for 'professional' gets you grovelling
Models over-apologise when told to be professional about bad news. You get four sentences of regret before the reader learns what happened. The fix is not a better adjective — it is a rule: 'no more than one sentence of apology'. Rules survive; adjectives get interpreted.
State the facts the reader will check
The old date, the new date, and the real reason. A model given 'there is a delay' invents a vague cause that will not survive contact with the client. Give it the actual one, and say what you are doing to protect what the client cares about.
Lead with the change, not the context
Ask for the subject line to carry the new date. People triage bad news in the inbox list, and a subject that says 'quick update' makes them open it expecting nothing.