Some emails not being received–please help!

I am trying to send out thank you emails via seamless donations and some are going out and being received perfectly but some are not. I have checked spam folders and the email did not go there. I added the Mandrill plugin but I obviously don’t know enough to figure out how to make it work. I am technology challenged so need someone to tell me in simple terms what I need to do to make this work. I know that the emails going to aol.com are not on the working list. Thanks!

Susan