According to RFC 561 ("Standardizing Network Mail Headers ", 1973) it was literally "at":
We suggest that the text of network mail, whether transmitted over the FTP telnet connection (via the MAIL command) or over a separate data connection (with the MLFL command), be governed by the syntax below:
Example:
From: White at SRI-ARC
Date: 24 JUL 1973 1527-PDT
Subject: Multi-Site Journal Meeting Announcement
NIC: 17996
At 10 AM Wednesday 25-JULY there will be a meeting
to discuss a Multi-Site Journal in the context of
the Utility. Y'all be here.
RFC 733 (Standard for the Format of ARPA Network Text Messages, 1977) has examples of addresses in both formats and this note: Whenever a message might be transmitted or migrate to a host on another network, full hierarchical addresses must be specified. These are indicated as a series of words, separated by at-sign or "at" indications.