Yes, it is possible to obtain a SSL certificate for dynamic IP based servers. For mail authentication you need a proper "MX" DNS entry. Please choose "HTTP" type authentication during CSR placement for servers with <subdomain>.synology.me, <subdomain>.dyndns.org or <subdomain>.myqnapcloud.com domain, as authentication emails won't be processed by synology.me. If you are running your synology disk station with your own domain, you can of course also use email validation.
Synology CAA DNS records are allowing us to issue PositiveSSL and RapidSSL certificates for the following domains:
You can NOT obtain SSL from us for the following domains:
Dynamic DNS with your own domain => Email validation:
For validating your SSL certificate via Email you need a valid MX entry for your domain or at least a valid email forwarder with your domain registrar. Allowed Emails are admin@, administrator@, webmaster@, root@, postmaster@. For validating .com/.net/.org you can also choose the email address from the WHOIS entry of your domain. Please be aware that this option is not available for some domain suffixes (e.g. .de, .at). The certificate authority will send an email with validation link that you will have to approve. Without approval the SSL cannot be issued!
Dynamic DNS subdomain (synlology.me, myqnapclud.com) => HTTP validation:
For servers on <subdomain>.synology.me, <subdomain>.myqnapcloud.com or <subdomain>.dyndns.org etc. please choose "HTTP" validation while placing your CSR. In this case the email validation won't be possible because emails sent to qnap, synology etc. won't ever be forwarded to you. - Of course you can use email validation if your synology diskstation is running on your own domain and can receive validation emails on root@, administrator@, ...
Please find further details regarding SSL installation in the Synology knowledgebase