While using an ACU Network, email accounts hosted on non-Google servers cannot be used to send email using a local client.
A local client is an application installed on the computer you're using, and they include the macOS Mail app, the Windows Mail app, Outlook, Thunderbird, etc. Sending email uses the Simple Mail Transfer Protocol (SMTP), which is blocked on ACU's network. When using a local client, email is sent directly from your computer. But if the email account has a web client that uses off-campus servers to send emails, that web client will still work because the email is being sent from the off-campus server.
The best workaround for this issue is to access your email through a web client using a web browser, e.g.: gmail.com, yahoo.com, etc.