If that file does not exist, the address also does not exist, and so the mail is bounced. If the file is 0 bytes long, the systemwide default instructions are used. The leading ampersand is optional if the address starts with a letter or digit. Forwards are not actually performed until after all other instructions in the file, and if one of the later instructions fails, the forwards are not performed. So if a temporary error is encountered, and later the mail is delivered successfully, the message is less likely to be forwarded twice.
Relative paths are relative to the home directory of the account controlling the address.
Relative paths are relative to the home directory of the account controlling the address. 99 succeeded, and further instructions will be skipped. Preceding forwards will still be performed.