Making your sendmail to respond on both 25 and 587 ports the easy way
Yes, you can do it very easy and fast if you have Webmin installed on your server.
Go to Servers -> Sendmail Configuration -> Sendmail Options and make sure that SMTP port options parameters are set to:
Name=MTA
Port=587, Name=MSA, M=E
Just like on the screenshot below:
Click "Save and Apply" button and you're done. Now your mail server will accept mail on both 25 and 587 ports.