Apology - Unreturned message

Business Letter

Dear …

RE: Unreturned Message

Please accept my sincerest apology for not returning your message of __(yesterday or date) __. That was less than professional of me and you have every reason to feel annoyed.

I usually return all of my messages before the end of each day. In this case, I had a bit of an emergency that prevented me from doing so, however, I do not wish to make excuses for myself and I want you to know that I am willing to accept full responsibility.

Please let me assure you that I will not make this same mistake twice. I only hope that you can accept my apology and allow me the opportunity to get our relationship back on track. Looking forward to hearing from you soon.