Got a great question this afternoon wondering whether sending e-mail via TCP port 587 instead of 25 would allow for a better deliverability. Does it make a difference? Does the SMTP bounce message "Proper Authentication Required) have something to do with this?
If your talking about bulk mail delivery, port 587 it’s not used for that. It’s used normally for personal one to one email.
ISP’s used to allow you to use port 25 on their mail server or mail servers that are not hosted on their network until the virus and spam issue exploded a few years back. So today, some Internet service providers have blocked access to SMTP port 25 for sending mail OUTSIDE of their network to curb viruses and spam when they try to send email from your computer at home. This means you can use their mail server (their email accounts) to send email, but if you 0wn3d (owned) your own domain and mail server outside of their control, you can’t send email to it on normal port 25. You have to use port 587 which is authenticated (hopefully since you set it up) to send email to it and then that server will deliver it to the receiptant via port 25.
Another example of using it is some hotels and other free Wi-Fi (Starbucks) also hijack (sniff/watch) port 25 connections leaving their network so they can check for spam or other virus laden issues. When those issues happen it’s their network that ends up in trouble. If you don’t like them reading your email, using port 587 to your mail server. They are of the same mind set that port 25 is to open and un-authenticated to send email on from their network,
See, it’s understood today that email clients (Outlook) or computers
shouldn’t be sending email in port 25. A mail server sends email on
that port. We are in essence saying mail servers will send email
on port 25 to each other, but when submitting a message to your mail
server for delivery to someone else or to a system not on the same network, use port 587.
out this article as well which explains the use of it.
Use TCP Port 587 For Mail Submission