Calendar Server Email Invitations

I went back and disabled SSL for the external email settings for the Calendar service, and it started sending mail. Not sure why that was an issue.

Emails are going out now, but I'm having more problems. I sent an invite to addresses at mac.com, gmail.com, yahoo.com, aol.com, and outlook.com. They all received the invite via webmail. AOL and iCloud didn't offer the option to accept. In fact, iCloud displayed a bunch of XML or something in the message body (see screenshot below). When I accepted in the other webmail interfaces, I didn't receive notification through the calendar server. Is it just not possible to get the calendar server working with external mail hosts/clients?

I'm using an external email server for calendar notifications. Do I have to use the built-in mail server in order for this to work?

When someone accepts or declines, the email makes it back to my external mail server. But, something isn't right. The address below is creating a folder in the mailbox I'm using for the calendar server. As you can see, there appears to be a folder for every external invite with an accept/decline message. Maybe this is why my calendar server isn't able to pick up the accept/decline messages?

ical+93d3e2d8-344a-4a01-a974-9f23f868e55e@domain.com

I suspect my external mail host doesn't support "plus addressing" as outlined in this Apple support article:

OS X Server: iCalendar Message-Based Interoperability Protocol (iMIP) support - Apple Support

I created a Gmail account, because I know they support plus addressing. I changed my calendar server to use the Gmail account, but the error log is showing login issues. I have double and triple checked the Gmail settings. Any suggestions?

2015-05-27 08:22:32-0500 [-] [caldav-0] [IMAP4DownloadProtocol (TLSMemoryBIOProtocol),client] [txdav.caldav.datastore.scheduling.imip.inbound.IMAP4DownloadProtocol#error] IMAP login failed for my_account@gmail.com