Be also sure to have configured correctly email.
Host field should be compiled like the following example:
Incoming Mail (IMAP) Server
Transport: Select transport as Gmail, Yahoo or IMAP
Host: Every email service has unique IMAP HOST like
Gmail
{imap.gmail.com:993/imap /ssl}INBOX
Yahoo
{imap.mail.yahoo.com:993/imap/ssl}INBOX
https://www.uvdesk.com/en/blog/setup-mailbox-uvdesk-open-source-helpdesk/