Date: __/__/____ (Date)
Subject: Apology for late response
This is with reference to the query sent by you on __/__/____ (Date) regarding _________ (mention query).
I am the _________ (designation) of the department and I would like to apologize for the late reply to your query. You have been our premium customer, and we really appreciate the loyalty and faith that you put in us. Due to ______ (heavy volume of transactions/multiple query/shortage of staff) we were unable to reply at the given time.
I assure you that this kind of mistake will never happen again. As per your query ___________ (resolution provided). I sincerely hope that you will consider our situation.
For any query or feedback, you can directly contact me at _______ (contact information).
____________ (Your name)
____________ (Contact details)