So, today I spent about 2 hours trying to setup an SMTP service sender (attempting to use AWS SES, Mandrill, MailGun, etc) under a DigitalOcean droplet.
Some weeks ago I had setup PostFix and friends, without success. I left it cool after being blocked because after double checking my configuration mail just would not get out.
Today I tried again using an external SMTP service... after some hours of tinkering I came to the conclusion that DigitalOcean blocks the standard SMTP outbound ( 25, 587 and 465) ports in their Droplets.
You can try this by using the great service http://portquiz.net/ Just do something like
telnet portquiz.net 25 telnet portquiz.net 587 telnet portquiz.net 465
And you will get a connection-timed-out error. But if you do
telnet portquiz.net 3123
Or any other random port, you will be able to connect.
Fortunately for my SMTP sending requirements, AWS and some other SMTP services provide non-standard ports so that messages can be sent.