iDRAC8: Alert emails not working, "RAC0225: Sending the test mail failed"

I'm trying to get the alert emails working on an PowerEdge R530 (IDRAC8, firmware 2.30.30.30).

What I've done so far:

1. In Overview -> Server -> Alerts:

  1.1. In "Alerts" tab: Enable alerts

  1.2. In "SNMP and Email Settings" tab: Enter IP address and port of mail server, disable authentication, click "Apply"

  1.3. In "SNMP and Email Settings" tab: Set an email address for "Email Alert 1", check the corresponding "State" checkmark, click "Apply"

2. In Overview -> iDRAC Settings -> Network:

  2.1. Set a "DNS iDRAC Name" and "Static DNS Domain Name", click "Apply"

Now, when trying to send a test email from Overview -> Server -> Alerts -> SNMP and Email Settings, I get an error "RAC0225: Sending the test mail failed."

The mail server doesn't log any connection attempts.

How can I debug this? I already Googled for two hours but none of the solutions worked :/