[Exim] doc micro-bug

    When `sender_verify' is set, Exim checks the senders of incoming SMTP
    messages, that is, the addresses given in the SMTP MAIL commands. This
    does not apply to batch SMTP input by default, but `sender_verify_batch'
    can be set true if it is required.

    The check is performed by running the same verification code as is used
    then Exim is called with the `-bv' option.
    ^
s/as is used then/as is used when/