Networking comes before application (Moodle and Mail). Since you've said you had a plugin that has been removed successfully and re-configured the smtp service back to what what it was ... are you sure that the Moodle can see the mail server itself ... at port shown.
Since it's linux, ssh into server and as root user of the system, install nmap ... a command line network scanner.
Once nmap is installed test the networking ... via this command:
nmap -P0 -p 465 [IPaddressorFQDNofMailServer]
IF nmap returns that port is closed ... not open ... then the issue very well could be on the mail server end.
'spirit of sharing', Ken