Install libopendkim-dev by entering the following commands in the terminal:
sudo apt update sudo apt install libopendkim-dev
Headers and development libraries for the OpenDKIM library
The OpenDKIM Project is a community effort to develop and maintain a C library for producing DKIM-aware applications and an open source milter for providing DomainKeys Identified Mail (DKIM) service. . This package provides the required header files and development libraries for developing against the OpenDKIM library.