Smooth over a hard-to-write email

I need to send a hard-to-write email and need you to phrase it tactfully without losing my position.

Recipient and relationship: (e.g., partner vendor / my client / a cross-department manager)
My core message: (state plainly what you want to say, e.g., "we can't make this delivery date," "I'd like a deposit first")
Background: (add some context)
Desired outcome: (how you want them to respond)

Write me an email:
1. Build goodwill at the start — don't drop the bad news right away.
2. State the hard part clearly, but respectfully, giving the other side an out.
3. Propose a concrete, workable next step or alternative.
4. Keep warmth in the relationship at the close.

Chinese letter format, neither overly servile nor too harsh — natural and tactful.