After installing MailCow, I was able to send emails to my own server and also receive emails. However, I was not able to send emails. I would like to share my steps to resolve β as this stumped me for a couple of days.
Checking the Postfix logs, I could see the emails were timing out.

I am using CloudFanatic as my VPS (Virtual Private Server).
I logged into my mail server to check my ports were open, and they were. Including port 25.
netstat -tunlp

Then I reached out to my VPS provider to ask if they were in fact blocking port 25 at a different level. CloudFanatic replied that they had opened port 25 for my IP and after that the issue was resolved.

I hope this may help other who are setting up MailCow and get stuck.
Best,