Why does authid have an extra AT @ symbol at end?

Why does the authid have an extra @ symbol at the end?

I see this in my maillog and when milters read the authid.

Dec 2 12:50:47 host sm-acceptingconnections[31928]: AUTH=server,
relay=c-xx-xx-xxx-xxx.hsd1.mn.comcast.net [xx.xx.xxx.xxx],
authid=xxxxx@xxxxx.org@, mech=LOGIN, bits=0

Thanks.

Jim