exim4-dev software package provides header files for the Exim MTA (v4) packages, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install exim4-dev
exim4-dev is installed in your system.
Make ensure the exim4-dev package were installed using the commands given below,
$ sudo dpkg-query -l | grep exim4-dev *
You will get with exim4-dev package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | |
| Package | exim4-dev |
| Version | 4.87-3+b1 |
| Description | header files for the Exim MTA (v4) packages |
| Sub Section |