courier-imap software package provides Courier mail server - IMAP server, 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 courier-imap
courier-imap is installed in your system.
Make ensure the courier-imap package were installed using the commands given below,
$ sudo dpkg-query -l | grep courier-imap *
You will get with courier-imap package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | |
| Package | courier-imap |
| Version | 4.17.2+0.76.3-1 |
| Description | Courier mail server - IMAP server |
| Sub Section |