Using Express to send verify email by SendGrid. Work Perfectly on localhost, but did not work on my droplets (Ubuntu 18.04.3 (LTS) x64)

I was working with Express and SendGrid to send email verification to my own Gmail. I tried on localhost it works perfectly. However, I did not receive any emails when I deploy and run on my droplet. Moreover, I did not receive any error messages. So I assume the program successfully sends the email but been blocked by some port. Therefore, I find this question https://www.digitalocean.com/community/questions/i-can-t-connect-to-any-port-25-i-ve-tried-yahoo-mail-server-so-i-can-t-send-mail-from-the-vps. I tried port 25 on my droplet it did not block. Any suggestions on why this is happening?