Voidpop is a dummy POP3 server. It accepts any username/password and always reports that there are no messages.
I found myself in a situation where I needed someone to send some emails via a particular SMTP account, but there was no corresponding incoming mail account. Their email client refused to even try to connect to the SMTP server if it couldn't first sync the mailbox from an incoming mail server. I didn't find any existing dummy POP3 or IMAP services, and POP3 looked simpler to implement.
You can connect to pop.voidpop.org on port 110 with any username and password,
and the server will report that you have 0 messages.
$ python3 -m venv env
$ env/bin/pip install voidpop
$ env/bin/voidpop --help