I use Thunderbird in conjunction with Fastmail. Fastmail allows one to create aliases for an email account i.e. [email protected] where domainname.com is one of many domains that fastmail allows users to use and “alias” is something you make up…
Any mail sent to that alias is actually sent to your Fastmail account. This allows you to receive and send using this [email protected] email address which can wind up in an email box of a domain you own and has configured in your Fastmail account.
Thunderbird allows you to set up these [email protected] for each email account if you want. I have a series of these set up for my Fastmail account in Thunderbird. I may have overlooked this but it seems eM Client does not have this alias capability?