An apology marketing email is sent to the client after you realised that major blunders have been made in running his/her marketing campaign that did not truly support your client’s products. The purpose of this email is to let your client know that you have accepted your mistakes and will make efforts to compensate him/her for the mistakes.
In the first paragraph of the email, you must clearly tell your customer that you are deeply sorry and regret the fact that you were unable to run his/her marketing campaign as you claimed.
In the second paragraph of the email, you must proceed forward and confess your mistake by explaining to him/her what has really happened. You should tell the client that there were some unwanted confusions that you failed to handle in time and it had a direct impact on his/her marketing campaign.
In the third paragraph of the email, you must explain to him/her how you are willing to compensate for the mistakes that you have made. Not to mention that you must show your keenness to continue business with him/her.