I have attempted to send emails using your smtp services but receive the smtp error 554 Invalid From Address.

Although authenticated using my dynu account credentials, my "From" email is nclark@xxxxxxxx.com which is a valid email account not hosted on the dynu domain. If the "From" address is changed to a dynu.com hosted email account then the mail is sent properly.

Is this some sort of protocol that you have put in place to prevent spam being sent from your domain?

I have a demo software application that needs to send email using a known email account that is not visible to the users. It will use the authenticated dynu account smtp services but the "From" email address will not be a dynu email account.

How can I get around this issue/restriction?

'554 Invalid From Address' looks like an anti-spam protocol on the server end like you mentioned.

I'm having the same issue. My testing so far has only been to gmail email addresses. It looks like gmail is rejecting my email as spam.

I'm using hmailserver with a xxxxx.dynu.com subdomain for mx and relay.dynu.com as a relay server.

I understand that I cannot add spf or dkim records to 3rd level subdomains at dynu.com, which would assist in whitelisting my emails. However, I was previously using a google apps account to relay mail and had no problems (other than I just want to get away from google). Mail was sent and received just fine.

Is this a setup issue by virtue of missing spf and dkim records? Or is gmail refusing to accept mail from relay.dynu.com?

I just discovered that emails from the domain I registered through the smtp relay service go through fine, but emails from other domains get blocked. I assume this is intentional.

Email service is domain name based to prevent spam. If you sign up for the other domains under the same account, you can use the same credentials to relay emails for all 3 domain names.

have the same problem. and only on smart host for 3 domains sending out. ...

The issue for me with this, was the commandline arguments that Postfix was passing to sendmail -- included the original sender email address as part of those, which is what Dynu was rejecting.