Description:
When i create an appointment in a shared calendar and add participants by simply
begin typing till i get a popup with a list of matching email contacts, select one,
and finally click at "save" button, then i get "syntax-error"  messages as notification message.
Details in "Actual Results".

Reproducible: Always

Steps to Reproduce:
1. Create an appointment.
2. Go to the participants tab and choose at least one.
3. Klick the save button.
4. A few seconds later the error message described above appears as notification message.

Actual Results:  
I get notification messages like the following example instead of getting an email sent:

Error ending a message. Message sending failed since the following recipients were rejected by the server:
"c.rauecker@vcnc.org" (The server responded: "5.5.2 Syntax error. r5sm3985840wjy.37 - gsmtp")
Jenning Sch?fer (The server responded: "5.5.2 Syntax error. r5sm3985840wjy.37 - gsmtp")

Expected Results:  
Email for each participant should be sent, an invitation, i think.

I experimented a little bit and found out 2 things:

1. per default, selecting a contact appearing in the popup/dropdown as described above,
    leads to a contact inserted in the textfield of the form: Buddy-Name <abuddy@myorg.com>
    or thelike. changing that string simply to the format: abuddy@myorg.com manually,
    then the invitation maill will be sent as expected.

2. the error appears no matter using a google-calender backend or a caldav backend.

Hope that helps you. Specially point 1 seems interesting.