some more info :
The products affected by this issue are, WC 7120/25, WC 74xx, WC52xx, and XC 550/560/570. All of these devices are out of production. Their replacements all support SHA256 and 2Kcertificates.
If you have a network trace of the failure you can see if the certificate is signed with sha256.
In Wireshark open the trace file.
Type tcp.port==xxxx into the display filter dialog box and click "Apply" (Where xxxx is the port the device is using for SMTP).
Approximately ten lines down in the top window you will see a line that has "Certificate" in the info field. Highlight this line and expand the first entry for "Secure Sockets Layer" in the middle window.
Continue to expand the Subtrees until you open one called "Certificates".
Now you will need to find the certificate that is being used for SMTP, usually it will have "smtp" somewhere in the "id-at-commonName="
Expand that certificate and look for the line that says "algorithmIdentifier" if it says "sha256WithRSAEncryption" next to it you will have a problem. Most devices with DMP controllers do not support this signature and the email will fail with a 017-714 error.