Frankfurt Airport is large, and there are a number of postal drops and a post office.
It’s probably better to go to the Web page and look for Deutsche Post.
It’s not well-organized, but there appear to be numerous drops in Terminal 1 (especially in/around Concourse B). There are two in Terminal 2 Concourse D. Given that Deutsche Post logo is black and yellow, you should be able to find the mail drops fairly easily.
There is a post office in Terminal 1, Concourse B First level. It’s open Monday-Friday from 7.00-19.00, Saturday and Sunday 11.00-18.00. It’s next to O2 and near a Travelex.
Last updated 16 July 2017