The supported IMAP email invitations require the server to have access to an IMAP server for incoming mail and an SMTP server for outbound mail.
The incoming IMAP email service must support "plus addressing". This refers to adding a plus (+) sign and a tag to the name portion of the email address to create a sub-address that the Calendar Server uses to recognize an invitation reply.
If invitation responses are not being acknowledged by the Calendar Server, please check with your ISP (Internet Service Provider) to verify that plus addressing is supported through their mail servers.
The IMAP account used for this purpose should be dedicated solely to the calendar server; other incoming emails will be discarded.