When you send outbound emails, Xeams acts as an SMTP client and therefore, the SSL certificate of Google plays a role. The Let's Encrypt cert you're using in Xeams does not play any role. Therefore, the problem is not related to the certificate you're using in Xeams.

Could you please post the communication between an outbound email to any gmail.com address. That will help us understand why is that happening. Use the following steps:

• Send an email to any address on Gmail.com
• Ensure you see that message in Live Monitor, confirming the message goes through Xeams
• Then, go to Tools/View Logs, select SMTPOutboundConversation.log and display its contents. Most likely, the last communication will belong to the test email you just sent.
• Post that conversation here.

An FYI: A few days ago we got a phone call from a user with similar problem. In that case, he was using a firewall from Untangle with SMTP filtering on. For some reason this firewall was removing the STARTTLS feature from EHLO making Xeams not to use encryption. What firewall are you using?