*** First off, I’d suggest upgrading from Ubuntu 18.04 to either 20.04 or 22.04 as 18.04 is pretty dated and may actually be “End of Life”. ***
Regarding your issue, to offer the best assistance please describe in more detail how you are going about sending email, whether it be within Usermin or an email client like Outlook.
Out of the box, Virtualmin itself is setup to send and receive email by default. So you shouldn’t have to do a great deal of extra configuration.
Easy enough. Build out a new instance on a 20.04.x. Install virtualmin. Created backups from the current, import those backups on the new one. Test everything using spoofed host entries in your local hosts. Once verified all is working as expected, flip DNS or what I usually do is swap IP’s from the old to the new & vice versa incase a reversion is required etc.
Cake walk!