[exim] Where the postmaster is checking??

Hello!

    I have two checks, first in acl_check_rcpt:
require verify        = sender/callout=40s,random,defer_ok
    Second in acl_check_data:
   deny  senders = :
         message = A valid sender header is required for bounces
         !verify  = header_sender/callout=mailfrom=postmaster@???

So, I haven't configured postmaster checking. But sometimes my host
refuses mail with next message:

550-Postmaster verification failed while checking <xxx@???>
550-(result of earlier verification reused).
550-Several RFCs state that you are required to have a postmaster
550-mailbox for each mail domain. This host does not accept mail
550-from domains whose servers reject the postmaster address.
550 Sender verify failed

So, when and why postmaster checking is done? Exim 4.54
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With respect,
    Boris