sendmail() do not have to set them in the control
argument.sendmailOptions(...)name=value or by
passing a list of such tagged values. However, only the ones
below are used in base sendmailR.sendmailOptions(), a list of all set options sorted by
name. For sendmailOptions(name), a list of length one
containing the set value, or 'NULL' if it is unset. For uses setting
one or more options, a list with the previous values of the options
changed (returned invisibly).