MFC r272288,272289:

When setting environment variables in the atrun script, use the
"export foo=bar" form instead of "foo=bar; export foo" since the
former allows the shell to catch variable names that are not valid
shell identifiers. This will cause /bin/sh to exit with an error
(which gets mailed to the at user) and it will not run the script.

Obtained from: OpenBSD (r1.63 millert)