[WPMU DEV Dashboard] Emails being sent but not received

This issue isn't necessarily related to WPMU, but to WordPress and WooCommerce. We have been trying to send emails to our customers confirming order processing, however for almost a month the emails have been being sent (according to the plugin Post SMTP) however the test emails are not being received by any email we send them to. I wonder if you have any insights regarding this issue, and if you can help me ensure the emails are being delivered.