According to: http://www.postfix.org/postconf.5.html, append_at_myorigin should not be changed.
Without seeing your logfiles, it's a bit hard to understand what's going on. Try setting verbose logging for the snmpd in master.conf " snmpd -v
smtp inet n - n - - smtpd
the man page
append_at_myorigin (default: yes)
With locally submitted mail, append the string "@$myorigin" to mail addresses without domain information. With remotely submitted mail, append the string "@$remote_header_rewrite_domain" instead.
Note 1: this feature is enabled by default and must not be turned off. Postfix does not support domain-less addresses.
Note 2: with Postfix version 2.2, message header address rewriting happens only when one of the following conditions is true:
The message is received with the Postfix sendmail(1) command,
The message is received from a network client that matches $local_header_rewrite_clients,
The message is received from the network, and the remote_header_rewrite_domain parameter specifies a non-empty value.
To get the behavior before Postfix version 2.2, specify "local_header_rewrite_clients = static:all".