[exim] errors by sender verifying

A great mail provider "r-online.de"
doesn't allow since yesterday our verify procedure for sender checking:

here our configure part:

 deny    message = $acl_verify_message
          sender_domains = ! partial-dbm;DBM/etc/always_accepted_senders
          !hosts = +relay_hosts
          !verify = sender/callout=60s,defer_ok

and now 1 error message:

host mail.uni-magdeburg.de [141.44.1.10]: 550-Callback setup failed while
verifying <xxxxxx@??? <mailto:xxxxxx@t-online.de>>
550-(result of an earlier callout reused).
550-The initial connection, or a HELO or MAIL FROM:<> command was
550-rejected. Refusing MAIL FROM:<> does not help fight spam, disregards
550-RFC requirements, and stops you from receiving standard bounce
550-messages. This host does not accept mail from domains whose servers
550-refuse bounces.
550 Sender verify failed


Do you know what is bas at our exim configuration ???

MfG M.Lottmann