Using dev server with limited inbound traffic, can't send emails

We are trying to run joomlamailer on our development server which is hidden from the public and only allows inbound traffic from specific IPs.

Sending our test emails works when we open up our server to all inbound traffic but when we limit inbound to certain IPs we get the error message "Error Bad Request" and the test emails do not get sent. All other functions work, though. Our API connects with our Mailchimp account ok.

We have tried whitelisting the IP associated with our Mailchimp API which I assume is us18.api.mailchimp.com along with other Mailchimp IPs but it still doesn't work.

Are there other IPs that need to be whitelisted to get the ability to send emails to work?

We are on Joomla 3.9.19 and Joomlamailer version 3.0.3. PHP version 7.3.13

Thanks