In many formal letters, the first word after the salutation (e.g. "Dear Sir or Madam,") is the pronoun I, which is always capitalized. However, I recently wrote a letter for which this is not the case:
thank you for ...
A proofreader warned me that I should capitalize "thank", which left me baffled, because it is within a sentence starting with "Dear", therefore I see no logical reason for that.
I am not a native English speaker, though, so I am unsure: should I capitalize the word after the comma or not?
PS: In case it matters, I am in UK.