Symbols like the “@” sign should not be used in SNMP community strings. This is due to some manufacturers’ devices using the “@” sign for special purposes.
For example: Cisco’s switches use the “@” sign in SNMP community strings for community string indexing for querying VLAN specific information from devices.
For more information on this, check out Cisco’s documentation on Community String Indexing:http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a00801576ff.shtml