Using gmail to send as another domain's email address

[The late XV105]

For many years I have used gmail both to collect e-mail sent to another address I own and to send email as if it were that address. This matters because the other address is simply a forwarder (so it has no mailbox of its own) and I prefer not to expose my gmail address when replying or initiating a new e-mail; nothing dodgy in the slightest, just that to "send from" the other address is more professional and associates it with a website of the same domain.

By definition of the other address only being a forwarder it has no login. I cannot log in to smtp.the_provider.com - only to the account that administrates this (and many other) forwarders. This means that to send e-mail appearing as if it is this account I need to use another provider's smtp server - and I chose smtp.gmail.com as it was the only one that was free and worked every single time I tried, anywhere in the world and without irritating me that 10 minutes ago I appeared to be in another continent (but all I have done is move buildings and changed to an internet connection that happens to be a VPN tunnel to a different continent).

Some months ago, Google clearly changed something because out of the blue all e-mails I tried to send this way from both the web client and Gmail app on my smartphone and tablet started to fail with a message that can be summarised as "the relay smtp.gmail.com rejected your login" - even though the credentials are correct and I have already verified that I own the non-gmail account I am pretending to be.

Inference is that I can no longer use smtp.gmail.com as a free and convenient mechanism to send non-gmail even though the setup process (which as mentioned includes verifying that I own the other account, as well as logging in to my gmail account with smtp.gmail.com set as my smtp server) works fine. This applies whether I do it as an alias or not.

Before I give in has any other PPRuNer solved this challenge in a way that can be described, please?

Interestingly I can still do as I wish when using eM Client on my laptop but this is not always an option.

TVM,
TLXV

[le Pingouin]

These days there's every chance your spoofed e-mail will be marked as spam or rejected due to the sender address not being linked to Google.

[The late XV105]

That's what I'd have thought too, le Pingouin but mail sent as the same "from" address via eM Client (the email application I run on my laptop and which also uses gmail as the SMTP server) has never - not even once - been rejected by the relay or bounced further down the line and I have never had anyone say they didn't receive what I sent or had it arrive marked as junk.

It is most odd that I can do this as described via laptop client yet not via Gmail web or Android app. Currently, the Gmail option to "send as" is as much use as a chocolate teapot in either of the latter two scenarios.