Formal greeting with title and surname
Example: Dear Mr. Thompson,
If you don't know the person or have their name, then Dear Sir or Madam,../ To whom it may concern,.. (open application)
Conventional grammar
Example: I am writing to inform you that I will be unable to make our appointment tomorrow morning.
Relative pronouns included
Example: Unfortunately, a situation has arisen and I need to take care of it.
Specific language
Example: I sincerely apologise and hope that this is not too great of an inconvenience for you.
Transitions
Example: Sincerely,../ Futhermore,../Therefore,...
Polite, deferential language
Example: I sincerely apologise