3.6 Checking Sender Address

A special language construct is provided for verification of sender addresses (callout):

on poll $f do
when success:
  accept
when not_found or failure:
  reject 550 5.1.0 "Sender validity not confirmed"
when temp_failure:
  tempfail 450 4.1.0 "Try again later"
done

The on poll construct runs standard verification (see standard verification) for the email address specified as its argument (in the example above it is the value of the Sendmail macro ‘$f’). The check can result in the following conditions:

success

The address exists.

not_found

The address does not exist.

failure

Some error of permanent nature occurred during the check. The existence of the address cannot be verified.

temp_failure

Some temporary failure occurred during the check. The existence of the address cannot be verified at the moment.

The when branches of the on poll statement introduce statements, that are executed depending on the actual return condition. If any condition occurs that is not handled within the on block, the run-time evaluator will signal an exception⁵ and return temporary failure, therefore it is advisable to always handle all four conditions. In fact, the condition handling shown in the above example is preferable for most normal configurations: the mail is accepted if the sender address is proved to exist and rejected otherwise. If a temporary failure occurs, the remote party is urged to retry the transaction some time later.

The poll statement itself has a number of options that control the type of the verification. These are discussed in detail in poll.

It is worth noticing that there is one special email address which is always available on any host, it is the null address ‘<>’ used in error reporting. It is of no use verifying its existence:

prog envfrom
do
  if $f == ""
    accept
  else
    on poll $f do
    when success:
      accept
    when not_found or failure:
      reject 550 5.1.0 "Sender validity not confirmed"
    when temp_failure:
      tempfail 450 4.1.0 "Try again later"
    done
  fi
done