Here are a few synonyms to express your disappointment when writing an email or a letter:

  1. regret
  2. distress
  3. discontent
  4. dissatisfaction
  5. displeasure
  6. despondency
  7. discouragement
  8. mortification
  9. letdown
  10. failure
  11. setback
  12. frustration
  13. gutted
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