Dear (Name of the candidate)
The purpose of this is to convey to you my sincere apologies for any inconvenience you may have experienced last month with respect to ____ (Cause of Inconvenience). We continue to be committed in reaching your expectations. Once again, Let me convey my sincere apologies concerning this and also would see to it that this would not happen in future.
Sincerely, (Your name and Your Digital Signature)