How to make server respond to e-mail verification requests

Request for help at May First People Link here: https://support.mayfirst.org/ticket/297

Hi Jamie,

You solved the problem of some e-mail servers (including an e-mail host of mine and another MayFirst member) requesting a verification of e-mail that one of the MayFirst servers wasn't responding to.

I knew I needed this for Stedile but I thought I could figure out what I needed to do from your e-mail. I was wrong!

I just figured out the problem. Our log says: Nov 9 17:40:16 harry postfix/smtp[15363]: E405E54057: to=, relay=example.com[12.12.12.123]:25, delay=62030, delays=62030/0.02 /0.35/0.13, dsn=4.0.0, status=deferred (host example.com[12.12.12.123] s aid: 451 Could not complete sender verify callout (in reply to RCPT TO command)) "sender verify callout" means that your server, before accepting our email, replies to our server to see if the sending email address is valid. We recently upgraded that server and I did not turn on smtp services since we don't need them (although the box does send emails). Since we don't have port 25 open on the box, the sender verify callout failed and your email was queued. I just fixed the problem, so you should get the emails now.

For the record, I think that sender verify callout is a really really bad idea. I could send your server 1,000 messages from joe@bob.com and your server would effectively do a denial of service attack against the bob.com server as it asked it over and over again if joe@bob.com is a legit email address. Yuck!

Resolution