I currently have dovecot and postfix set up, and I can log in locally because rainloop web mail works perfectly fine for both sending and receiving. However, all external login attempts simply fail, even with the correct username and password. How can I allow external logins(any config file changes, etc.)?
Please let me know about any error logs that might help, and I'll add them to this question.
I am not sure about what you mean when you say "External login",
but if you want to connect from a Mail client as Outlook for example, you should follow the steps of this tutorial : https://support.rackspace.com/how-to/dovecot-installation-and-configuration-on-centos/
Then you would be able to connect Outlook (or any other mail client) to your local Postfix mail directory from any server that is connected to the same local network
Hope it helps !