In another thread (https://forum.emclient.com/emclient/topics/reply-to), it’s claimed that email aliases can be used to set reply-to addresses. If that’s the case then it doesn’t seem to be working properly, at least not with a custom domain.
If I have address [email protected], and I create alias [email protected], and choose the alias when sending email, the email will get [email protected] as both from: and reply-to: headers. It should have from: [email protected] as and reply-to: [email protected]. Current implementation seems to confuse gmail for example: mail sent with alias takes longer to deliver and is likely to be dropped entirely. Mail sent with ISP address gets through fine.
I think that the alias option is dependent on your provider.
The thread you quoted is what used to happen with some providers like Gmail. I don’t know if it still does as that was some years back that I made that comment.
With some other providers like GMX, it will work as with your custom domain as long as you already have the alias setup using the web interface for that provider. If you haven’t, it will reject the message completely. I think most providers do that now, as relaying (sending from a different address to the one used to login to the server) is a tool most used by spammers.
Your provider may allow sending from any address on that domain as long as you login to their server with a pre-authorized email address. Best would be to ask them what they provide.
In any case eM Client should provide some way to set the reply-to field. I don’t think this counts as relaying either as the from: address is the actual ISP address. This kind of scheme works just fine with Thunderbird and other email clients.
It’s not a problem with ISP either, it will happily send the mail (with both from and reply-to set to alias), why wouldn’t it? ISP SMTP server is authenticated after all, so there’s little risk of anonymous spamming. Problem is with gmail getting suspicious (and not without reason) with email sent from isp.com but with neither from: or reply-to: pointing to isp.com.
This is clearly either a bug or missing feature in eM Client.