We have been using our custom SMTP server through Zoho for a long time now and we recently updated our SSL certificate, after a failure of the server, and a the creation of a new exchange server. I am able to send mail normally through both Thunderbird and GMail using these same exact server settings. Since the certificate was renewed, when we try to send mail through Zoho we are receiving the following error message:

Reason:sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

I then go to Email Settings > Send Mail as > edit the email account, enter the correct password and receive this error message:

Invalid Security Certificate for the SMTP Server.

We have not made any other changes besides renewing our SSL certificate. Do you need to apply the new certificate to your internal systems to get this working again?

I found two different forums posts on Zoho support for this same problem that were both resolved by a Zoho technician updating the SSL certificate on the internal Zoho systems:

https://help.zoho.com/portal/en/community/topic/invalid-security-certificate-for-the-server-try-changing-the-incoming-port-to-25-without-and-try-again

https://help.zoho.com/portal/en/community/topic/suddenly-i-can-t-send-mail-due-to-pkix-path-building-failed

Our server settings:

Send mail using: Custom SMTP