Email won't send

When I try to send invoices by email this message is coming up: An error occurred while attempting to establish an SSL or TLS connection. The server’s SSL certificate could not be validated for the following reasons: • The server certificate has the following errors: • The revocation function was unable to check revocation for the certificate.
The test email worked fine.

Had the same problem and it was (is) caused by Norton 360 Email scanning feature.
Disabling the Email scanning (temporarily) allow the emails to be sent.

Related issue: Email revocation certificate error - #14 by Marlene_Coetzee but that thread is too old to bump/doublepost.