email_response probe is working fine for SEND metric. ROUNDTRIP metric is either missing for the same time stamps or is showing NULL values.
There is a rule on the mailbox to archive email sent by the probe for troubleshooting purposes.
The rule to archive email is set up as a "move" rule.
Changing the archive rule to "copy" allows email to stay in the inbox. The probe is then able to read the email to process data.