[exim] require verify = sender description

Hi,

how does verify = sender work. I can't find anything that says what
it does exactly in the docs. I'm seeing a valid sender get rejected
several times before being finally accepted. I added a warn line
in the config :-

warn    ! verify = sender
        logwrite = oops sender verify failed : $acl_verify_message ($sender_verify_failure)

require log_message   = require sender fails
    verify        = sender

but all I get in the message is for a routing failure e.g. :-

2017-04-07 13:13:48 oops sender verify failed : Sender verify failed (route)

I _think_ the issue is DNS timeout related but can't see any way to
increase the timeouts (apart from editing the code).

Regards,

D
lists/exim/users/2017-04-07.tx                                 exim-users
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