Users can send emails, but some external emails don't reach them. The admin can see the emails have been blocked but there is no way to restore or approve them.
These emails are blocked by the Greylist filter. This process happens at the SMTP level, so these emails will never reach your email server, making them impossible to be viewed by your users or be approved by an administrator.
Since this behavior is by design, the possible workarounds are:
- Add the sender's email address in the server's Global Whitelist, or
- Add the sender's email address in the Greylist exclusion, or
- Add the sender's email address in the recipient's Personal Whitelist.
It is always advisable to add the email address first in the Whitelists or in the filter exclusions list.