With regards to mail sending failed. Please goto Setup-->Data Administration-->System Log and check if you have any error message for Mail sending Failed.
Error: Could connect to SMTP host via the port
The host seems to be unreachable via the selected port, please make sure the mentioned port is open for our IP addresses. You can view the list of IP addresses from this link
Error: Could not find SMTP host
The host seems to be incorrect, please make sure you have mentioned a FQDN or the public IP address of the mail server.
Error: Authentication Failed
We are not able to connect to your mail server using the specified credentials and you would need to double check the credentials mentioned or reset the password and update the same. If you have MFA enabled in your mailbox, please use app specific password.