No. 587 is the submission port, used by your clients to send mail to you. It is usually authenticated.
You have no control over the rest of the world, and to the rest of the world SMTP is port 25. If you don’t have port 25, you don’t have a mail server. You’ll have to use a relay. Some cloud providers allow incoming but not outgoing, in which case you could receive mail with this VM, but would need a relay to send, but I think all the majors block both in and out.
For my VMs and containers in Google Cloud or AWS, I usually use Mailgun as my relay. It’s cheap(ish), and works well. But, there are many other options: Amazon SES, Sendgrid, Mailjet, etc. Many to choose from.
And, it has nothing to do with CentOS. Your hosting provider is blocking port 25. No operating system can make port 25 open, if it is blocked. Only your hosting provider can do that. If Microsoft providers a method to request port 25 be opened, then you could do that. I don’t think most of the big cloud hosts will do that, though.
If Azure is blocking port 25 to prevent spam, there is NOTHING you can do about it if they don’t allow outbound or inbound port 25 requests. I did a quick search and it doesn’t look like Azure is a hosting platform as such. What I read says it is more of a development platform.
I’m not postfix expert but I doubt it would start if there was no listening port assigned. The programmers would probably flag that as “FATAL” and not let the program start unless corrected.
The Azure platform will block outbound SMTP connections on TCP port 25 for deployed VMs. This is to ensure better security for Microsoft partners and customers, protect Microsoft’s Azure platform, and conform to industry standards.
If you’re using a non-enterprise subscription type, we encourage you to use an authenticated SMTP relay service, as outlined earlier in this article.
To fix this problem use ZOHO my email server was working and then one day my ISP closed up a hole I was using to send email via their email server even though they were blocking the port so to speak their mail server was allowing me to relay mail. They were not even aware that they closed it it took a bit to get down to the bottom of things because I was technically relaying through their server and direct sends were not supposed to work which they did not but all in all not a lot I could do about their new policies other than find another ISP. But truth be told I came across a website after a bit of searching that provides a free email that will integrate with your domain URL that website being ZOHO so you just select an email with aliases email@example.com and you can interface with that. And it all works out pretty clean actually a great website and service obviously for them to offer this for free I believe they need our support as well so please do purchase a service or support their work if you can support it and keep it free for those of us that need it.
Hope this helps interface with ZOHO and your good to go. Your options are limited you can pay Gmail a crap ton for the same service that ZOHO offers for free if you prefer. Its all up to you really personally I can’t thank these guys enough I have a business and I would have considered closing down cause my business was far to small to justify costs of email and hosting etc… Now I host and use ZOHO and can do so and keep my business open if I ever make it big enough these guys are going to get a good donation!
Maybe better yet I wonder if somehow Virtualmin could work with ZOHO to some how use their services integrated into their software somehow that would be rather cool.