This is a product limitation of IMSS.
Retrieving mails via POP3 is not initiated by your internal mail server but by the end users’ machines, which is different when relaying mails from SMTP.
A POP3 connection will close after all new POP3 mails have been processed. There is no way for IMSS to actively build connections to the machines as end users’ machines do not allow ports to receive POP3 mails. Thus, IMSS cannot release the quarantined POP3 mails to end users. This differs from SMTP mails because the internal mail server is attentive to the SMTP port.
As a workaround for the issue, retrieve quarantined mails for end users from the WebUI.
- Click Mail Areas & Queues > Query.
- Type the query criteria.
- Go to Display Log and select the timestamp corresponding to the mails you want to release.
- Click Download to get the mail file in EML format.
- Send the mail file to the end user.