This setup is the “modern” way to send mail from a custom-domain alias inside Gmail while using ImprovMX as the outbound SMTP provider, so your messages can be properly authenticated and less likely to get treated as suspicious - improvmx.com 
Unlike “Sending from Gmail” (which sends via Google’s SMTP), this method configures Gmail to send through `smtp.improvmx.com`, using credentials you create in ImprovMX - improvmx.com
# Prerequisites
You must be a paid ImprovMX subscriber to use ImprovMX SMTP - improvmx.com
This guide assumes a personal Gmail account; Google Workspace accounts may behave differently depending on organisational restrictions - improvmx.com
You should already have your incoming mail forwarding working (aliases forwarding into your Gmail inbox), because Gmail will send a verification email to confirm you own the address - support.google.com
# Create SMTP credentials in ImprovMX
Sign in to the ImprovMX dashboard, open your domain settings, go to “SMTP Credentials”, and add a new SMTP user by choosing a username (usually the full alias email address) and a password - improvmx.com
Keep that password safe, because Gmail will use it to authenticate when it sends mail through ImprovMX - improvmx.com
# Add DKIM and DMARC for SMTP sending
ImprovMX SMTP requires you to publish DKIM and DMARC records for your domain, which you can view in the ImprovMX dashboard after you have created at least one SMTP credential - improvmx.com
Add the provided records at your DNS provider, then wait for propagation (it can be quick, but can take up to 48 hours), and use the ImprovMX dashboard or inspector tools to confirm they are visible - improvmx.com
# Add the alias to Gmail as a sending address
In Gmail on the web, open Settings, then “See all settings”, then “Accounts and Import”, and under “Send mail as” choose “Add another email address” - support.google.com
Enter your name and the alias email address you want to send from, and make sure “Treat as an alias” is unchecked - improvmx.com
When Gmail asks for SMTP details, use these values: SMTP server `smtp.improvmx.com`, port 587, secure connection TLS, username is the alias address, and the password is the SMTP credential password you created in ImprovMX - improvmx.com
# Confirm the address
Gmail will send a verification link or code to the destination inbox that your alias forwards to, so open that message and complete the confirmation step - improvmx.com
If you cannot find the verification email, check spam and search for Gmail’s send-as verification message - support.google.com
# Test and use it day to day
Compose a new message in Gmail and choose your alias in the “From” dropdown, then send a test email to a different recipient address (not the same Gmail inbox your alias forwards into) - improvmx.com
If you want everything to send from that alias by default, set it as Gmail’s default “From” address in the same “Send mail as” settings area - support.google.com
# Troubleshooting notes
If Gmail reports it cannot reach the server or TLS negotiation fails, re-check the port and security selection, and confirm you used `smtp.improvmx.com` with TLS on 587 - support.google.com
If sending works but deliverability is poor, confirm DKIM and DMARC records are published correctly and have propagated, because those records are part of the required SMTP sending setup with ImprovMX - improvmx.com