This setup lets you receive mail on a custom-domain address (an ImprovMX alias that forwards into Gmail) and also send mail *from that same alias* inside the Gmail web interface using Gmail’s “Send mail as” feature - improvmx.com 
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If you want more reliable deliverability and proper authentication, the ImprovMX recommendation is to use their SMTP sending service instead of Gmail “Send mail as” - Gmail with ImprovMX SMTP
# What you get
You keep using your normal Gmail inbox, but you can choose your alias in the “From” dropdown when composing, so replies and new emails appear to come from your custom address (for example hello@yourdomain) - support.google.com - improvmx.com
# Before you start
You need your alias already forwarding into your Gmail inbox via ImprovMX, because Gmail will send a verification code/link to that inbox during setup - improvmx.com
You also need Google 2-Step Verification enabled, because the guide uses a Google App Password (not your normal Gmail password) - support.google.com - support.google.com
# Step by step setup
1. Turn on Google 2-Step Verification for your Google account, if it is not already enabled - support.google.com
2. Create a Google App Password and copy it somewhere safe, because you will paste it into Gmail’s SMTP settings during the wizard - support.google.com
3. In Gmail (web), open Settings, then “See all settings”, then the “Accounts and Import” tab, and in “Send mail as” choose “Add another email address” - support.google.com
4. Enter your alias email address (the address that forwards via ImprovMX) and the sender name you want, and untick “Treat as an alias” - improvmx.com
5. Enter the SMTP settings as follows, leaving TLS enabled.
SMTP server: smtp.gmail.com.
Port: 587.
Username: your full Gmail address (including @gmail.com).
Password: the App Password you created earlier - improvmx.com
6. Confirm ownership by opening the verification email Gmail sends you and completing the confirmation step - improvmx.com
# Testing and day to day use
Compose a new email in Gmail, open the “From” dropdown, pick your alias, and send a test message to a different recipient address (avoid sending to the same Gmail address your alias forwards into, because that can create a loop) - improvmx.com
# Troubleshooting notes
If the alias cannot send, double-check you used smtp.gmail.com, your Gmail address as the username, and a Google App Password (not your normal password), and try a fresh browser session (incognito or another browser) - improvmx.com
If messages land in spam or show warnings, make sure “Treat as an alias” is unchecked, avoid replying to yourself via the same forwarding path, and confirm your SPF record is correct and has propagated - improvmx.com
If you click Send and nothing happens, Gmail may be dropping the message due to loop detection when you send to your own forwarded address; try a different recipient for the test - improvmx.com
# Limitations you should know
ImprovMX explicitly warns this Gmail method relies on older Google functionality that could be discontinued, and it lacks proper modern authentication (SPF, DKIM, DMARC), so some providers may reject or heavily spam-filter messages sent this way - improvmx.com
You may see a “via Gmail” label on sent messages when using this method, and the guide notes it is not removable - improvmx.com
If you want more reliable deliverability and proper authentication, the ImprovMX recommendation is to use their SMTP sending service instead of Gmail “Send mail as” - improvmx.com