If you are using SSL/TLS for email access, you may receive an error message such as: "Mail can't verify the identity of the server <your_domain>
The certificate for this server is invalid [...&etc]"
This will happen if you don't have a premium "wildcard" SSL certificate installed for your domain which covers the mail. sub-domain.
In this situation the mail connection relies on the shared SSL certificate installed on the server, which as it is shared, can't be verified for your domain.
To "fix" this, you will need to do one of the following: