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Old 25 May 2022, 12:40 AM   #2
BritTim
The "e" in e-mail
 
Join Date: May 2003
Location: mostly in Thailand
Posts: 3,038
I am not completely certain that I understand your current setup. Assuming
  1. you have a Fastmail account associated with each yyy.com email address; and
  2. xxx.com is a domain administered by a Fastmail account.
You just define domain aliases for each xxx.com address, directing the messages to be delivered to the appropriate yyy.com account. Although the messages are delivered from name@xxx.com, you simply do not define a sending identity for name@xxx, and messages will automatically be sent out from the default name@yyy.com email address. Note, however, that the fact that your correspondents have sent to you as name@xxx.com will still be visible in your reply. Only the address you use to send the reply is name@yyy.com (along with any further replies by the original sender).

If xxx.com is not administered by a Fastmail account, it will be necessary to use forwarding in a way that will differ from one mail service to another, and you might also need to be careful about possible message delivery issues that can these days arise when using email forwarding.

If you describe your current setup more clearly, we can probably give you better advice.
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