Editing Anti-spam techniques (section)

===SMTP callback verification===
{{Main|callback verification}}

Since a large percentage of spam has forged and invalid sender ("from") addresses, some spam can be detected by checking that this "from" address is valid.  A mail server can try to verify the sender address by making an SMTP connection back to the mail exchanger for the address, as if it were creating a bounce, but stopping just before any email is sent.

Callback verification has various drawbacks:  (1) Since nearly all spam has forged [[Bounce address|return address]]es, nearly all callbacks are to innocent third party mail servers that are unrelated to the spam; (2) When the spammer uses a [[DNSBL|trap address]] as his sender's address. If the receiving MTA tries to make the callback using the trap address in a MAIL FROM command, the receiving MTA's IP address will be blacklisted; (3) Finally, the standard VRFY and EXPN commands<ref>{{cite web |title=VRFY command—Verify whether a mailbox exists on the local host |date=16 June 2017 |url=https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.2.0/com.ibm.zos.v2r2.halu001/smtvrfy.htm |publisher=IBM |access-date=19 December 2018}}</ref> used to verify an address have been so exploited by spammers that few mail administrators enable them, leaving the receiving SMTP server no effective way to validate the sender's email address.<ref>{{cite web|url=http://www.spamhaus.org/news/article/722/on-the-dubious-merits-of-email-verification-services|title=On the dubious merits of email verification services|website=www.spamhaus.org|url-status=live|archive-url=https://web.archive.org/web/20150908093827/http://www.spamhaus.org/news/article/722/on-the-dubious-merits-of-email-verification-services|archive-date=2015-09-08}}</ref>