The EX_USAGE error code (value 64) means that a command or configuration line was used incorrectly.

For instance, the sendmail daemon can be executed only by root, or by the person defined by the TrustedUser option (TrustedUser). If anyone else tries to run it with -bd or -bD, the following error is printed and sendmail exits with an EX_USAGE error:

Permission denied (real uid not trusted)

Clumsy fingers can also cause problems. If a command-line switch is used that is unknown to sendmail, the following error will be printed and sendmail will exit with an EX_USAGE code:

sendmail: illegal option -- bad option here

When an address is parsed, that address can be rejected if it contains control characters:

Address contained invalid control characters

If the EightBitMode option (EightBitMode) is specified with a bad character, the following error is printed and sendmail exits with an EX_USAGE code:

Unknown 8-bit mode bad character

If the DeliveryMode option (DeliveryMode) is given a bad mode specification with a bad character, sendmail will print the following message and exit with an EX_USAGE code:

Unknown delivery mode bad mode here