Certificate error during the connection to the email account via email client application
Cause
Mail is secured with a certificate which does not contain the information for other domains.
Resolution
If the mail server is secured using an SSL/TLS certificate issued from a particular domain, the domain name must be specified in the email client settings as the incoming/outgoing mail server, for correct sending and receiving of emails.
In order to resolve the issue change the certificate for securing email server with the one which issued for the server's hostname hostname.com:
Step1: Go to the Home > Tools & Settings > SSL/TLS Certificates > Let's Encrypt
Step2: Put hostname.com into the Domain name filed and press Renew.
Step3: Go to Tools & Settings > SSL/TLS Certificates . Click Change near the Certificate for securing mail field and select Lets Encrypt certificate (server pool) and click OK.
Step 4: Specify mail server hostname in Incoming mail server and Outgoing mail server (SMTP) fields in a mail client instead of domain's name.
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