[Exim] Underscore in domain part (rcpt to phase)

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I currently have a problem forcing exim to accept mail to addresses like

user@host_p.domain.tld

I have allowed the use of underscores in the helo but now it fails
during the rcpt to phase

When issueing a rcpt to:<user@host_p.domain.tld>

Exim answers with:

501 <malformed address: _p.domain.tld> may not follow user@host

It seems that the address gets cut at the _ (underscore)


I have turned off all syntactical checks in the acl_check_rcpt


Any ideas


Thanks


Ronny


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