Welcome @phonesrescue Happy you are here
Yes but its best to have separate certificates for both cases.
To issue custom mailserver ssl certificate go to https://SERVER_URL/websites/phonesrescue.co.uk/mail.phonesrescue.co.uk and add ssl
To secure multiple subdomains with same ssl certificate? phonesrescue.co.uk and mail.phonesrescue.co.uk
What you need is a Wildcard ssl certificate.
Check this out to add wildcard ssl certificate for your website.