The 602 error is a generic failure notice and you'll need to log on to SmarterMail as a system administrator,  navigate to Manage>Troubleshooting>View Logs to search the Delivery log for this attempt in order to find the underlying error that prevented delivery. 

Turning off "Enable TLS if supported by the remote server" located inside the domain's settings (under the Options card, Security) resolved this for me.

Not sure why.

I think the issue is because the recipient's SSL Certificate is expired. The customer wants all TLS turned off for their domain because they don't want to deal with it. [Facepalm]

You should be able to turn TLS usage off on a per-domain basis in our latest release in the domain's settings on the Security card at least.  Not to say its a good idea, but it IS doable :-P

Getting shit load of these...

Error: 602 Failed to connect to the recipient's mail server.  No MX records were found for the 'i-invest.se' domain. Status: 544 5.4.4 Host not found (not in DNS).Attempted to send the message to the following IPs:      77.111.240.13

Looking forward to seeing this resolved.

Just seen this!

For me the message is

"Certificate ChainStatus has returned a non empty array."

-- EDIT --
The "solution" given above as per on a domain basis has worked