1. Ho many maximum sub-domains can use this SSL, is there any quantity for per-domain?
2. Is there anyway can use this SSL for mail.DomainName it's also consider as sub-domain.
So far securing the mail server for multiple domain names (known as SNI) is a feature that it is not implemented in Plesk because the mail server does not support it: Is there SNI support for SMTP/IMAP/POP3? However, you can secure the mail server by following this article: https://support.plesk.com/hc/en-us/articles/115000179934-How-to-secure-mail-server-with-Let-s-Encrypt-certificate-.In regards to your other request, it is possible to access emails via mail.example.com instead of webmail.example.com. Please refer to the following article: How to set up an access to webmail over mail.example.com.
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For faster and more reliable delivery, add firstname.lastname@example.org to your trusted senders list in your email software.