STEP 1. Make sure you have 2FA enabled…
For this method to work, you need to have two factor authentification enabled for your Google account. If it’s not, follow the link below and set it up → Enable 2FA in your Google account…
STEP 2. Also in your Google account, create an App Password for Mail → Create an App Password… (You will copy and use this password later along with your Gmail address in the Google SMTP server settings “Add another email address” form)…
Select “Mail” for the app. Select your computer as the device. Hit “Generate”. Copy and keep the generated password for later in this process.
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STEP 3. Then, back in Gmail, add your Cloudflare-routed email address…
In Gmail go to Settings → Accounts → Send mail as. Under that section, click on Add another email address and fill in the first form with your name and your Cloudflare-routed email address. Untick “Treat as an alias” and click Next Step >>
STEP 4. Fill in the next form…
SMTP Server: smtp.gmail.com Port: 587
Username: your gmail address (incl. @gmail.com)
Password is the App Password you generated in Step 2.
Leave TLS enabled
Click Add Account >>
You will receive an email from Gmail asking you to confirm ownership with a code. Enter the code in the dialog (or click the link provided in the confirmation email), and you are all set!
Now you can select your new email address in the list when you compose a new message in Gmail. And, when reply to an email you received at your new address, the new address should be pre-filled in the From: field.