Here's how callbacks work. If you refuse them, I can't accept your mail: Connecting to smtp5.domain.tld [10.10.10.10]:25 ... connected 220 smtp.domain.tld ESMTP HELO mail1.merlins.org 250 smtp.domain.tld Hello magic.merlins.org [209.81.13.136], pleased to meet you MAIL FROM:<> 250 2.1.0 <>... Sender ok 553 5.5.3 ... Invalid RSET 250 2.0.0 Reset state MAIL FROM:<> 250 2.1.0 <>... Sender ok RCPT TO: 250 2.1.5 ... Recipient ok QUIT If you have an unfortunate server that accepts all RCPT Tos: Connecting to mx2.mail.yahoo.com [67.28.114.36]:25 ... connected 220 YSmtp mta214.mail.dcn.yahoo.com ESMTP service ready HELO mail1.merlins.org 250 mta214.mail.dcn.yahoo.com MAIL FROM:<> 250 null sender <> ok RCPT TO: 250 recipient ok QUIT But if you are a weird server, like yahoo, that accepts all RCPT Tos, and yet refuses the ones of known spammers, I should have disabled random callbacks so that I can check for those spammer addresses: Connecting to mx3.mail.yahoo.com [64.156.215.7]:25 ... connected 220 YSmtp mta241.mail.scd.yahoo.com ESMTP service ready HELO mail1.merlins.org 250 mta241.mail.scd.yahoo.com MAIL FROM:<> 250 null sender <> ok RCPT TO: 250 recipient ok QUIT Callbacks in action: mail from: police@fbi.gov 250 OK rcpt to: marc@merlins.org 550-Verification failed for 550-Called: 204.11.0.66 550-Sent: RCPT TO: 550-Response: 550 : Recipient address rejected: This service is temporarily unavailable. Please contact the recipient via other means. 550 Sender verify failed or mail from: xdkpelaqc@madtui.com 250 OK rcpt to: marc@merlins.org 451-could not connect to ns.madtui.com [8.7.146.81]: Connection refused 451-Could not complete sender verify callout for . 451-The mail server(s) for the domain may be temporarily unreachable, or 451-they may be permanently unreachable from this server. In the latter case, 451-you need to change the address or create an MX record for its domain 451-if it is supposed to be generally accessible from the Internet. 451 Talk to your mail administrator for details.