This is going to get very specific, and you are going to have to post exactly what the original address was, the heade5r of the email you are replying to and the details of what the from line in the forward is showing for anyone to even begin to guess what the issue is.

You appear to be aware of the identity feature and how it is supposed to work, but that process requires that the original information in the header is valid. So one off throw away addresses tend to fail the identity process because they are not configured.

Likewise for mail to go out using an address requires an SMTP server to be configured in the outgoing server (SMTP) for that address. The days of using a single SMTP server entery for multiple email addresses are basically gone. Even if your provider will allow it, your mail will probably end up in a spam folder at it's destination.

So can you please post the message headers of one of these emails? Suing the Mozilla pastebin is my preferred method for these type things as the forum mangles them. Just be sure to extend the retention for 3 weeks, not the default hour that will probably expire before it can be looked at. https://pastebin.mozilla.org/

Thank you for answering. I understand my approach is unusual, I've never had any spam problem, but I will rethink how I do it. My problem is dealing with many different providers with their space limits etc

Any way this is an example of email that was originally written for my gmail account, automatically forwarded to my domain account. The header is wrong and it is what Thunderbird sees. Apple Mail is dealing this correctly.

https://pastebin.mozilla.org/JhNpaMzg