TL;DR
"Send mail as" problems are almost always SMTP settings (authentication failed), routing (the verification email never arrives), or DNS alignment (the "via" label and spam). Fix the record that is wrong - or let SendMailAs configure SMTP and aligned SPF/DKIM/DMARC for you.
Quick diagnosis: which one are you hitting?
- Error the moment you click Add → Authentication failed
- Address added, but the confirmation email never arrives → Stuck on the verification email
- It sends, but recipients see "via" → The "via" label
- It sends, but everything lands in spam → Lands in spam
"Authentication failed" when you add the address
This means Gmail reached your SMTP server but the login was rejected. Work through these in order:
- Host and port. Use your relay's SMTP host with port
587and TLS (Gmail calls it "Secured connection using TLS"). If 587 is blocked, try465with SSL. - Username. It is the exact SMTP username your relay issued - often not your email address. Copy it verbatim.
- Password. Use the SMTP password / API key from the relay, not your Gmail or account password.
- Verified sender. Many relays refuse to send until you have verified your domain or From address in their dashboard. Do that first.
The verification email never arrives
Gmail sends a confirmation code to the address you are adding. If it never shows up, your receiving side is not set up - the address can send but nothing routes mail back to you.
- Confirm you have an MX / forwarding rule that delivers mail for that address to your inbox (Cloudflare Email Routing, for example).
- If you use a catch-all, make sure it is enabled and points to the Gmail you are setting this up in.
- Check spam, and give DNS a few minutes to propagate.
The full receiving-plus-sending setup is walked through in the Cloudflare Email Routing + Gmail Send As guide.
It sends, but shows "via" or "on behalf of"
This is the most common complaint, and it is a DKIM alignment problem. The server that sent the mail (your relay) has a different domain from your From address, and there is no DKIM signature on your own domain to tie them together - so Gmail shows "via relaydomain.com".
The fix: publish a DKIM key on your domain and have the relay sign with it, so the signing domain matches your From domain. Once DKIM is aligned, the "via" label disappears.
Setting aligned DKIM up by hand means generating keys, adding the right DNS records, and configuring the relay to sign with them. SendMailAs does this alignment automatically - see the permanent fix below.
It sends, but lands in spam
If your mail sends but hits spam folders, authentication is incomplete. Recipients' servers want all three records, aligned to your domain:
- SPF - authorizes your relay to send for your domain.
- DKIM - signs the message with your domain's key (same fix as the "via" label).
- DMARC - tells receivers what to do and ties SPF/DKIM to your From domain.
A deeper walkthrough of each record and the common mistakes is in why your emails land in spam.
The permanent fix: stop fighting DNS
Every problem above traces back to the same root cause: "Send mail as" needs an SMTP relay and SPF, DKIM, and DMARC records that align to your domain - and any one of them being off breaks sending or lands you in spam.
SendMailAs sets all of it up for you. It is built specifically for custom-domain send-as in Gmail:
- Email authentication, DNS records, and SMTP credentials are configured automatically in the background - no DNS expertise needed. About 5 minutes.
- SPF, DKIM, and DMARC are set up with DKIM aligned to your domain, so mail lands in inboxes and the "via" label is gone.
- No migration, no new inbox - it all happens inside the Gmail you already use. First domain free, $29/year for unlimited domains.
Frequently asked questions
Almost always wrong SMTP settings. Gmail is trying to log in to the SMTP server you entered and being rejected. Check the SMTP host and port (usually 587 with TLS), that the username is the full login your relay gave you, and that the password is an app password or API key - not your account password. If your relay requires the From address to be a verified sender, verify the domain there first.
Because the domain that actually sent the message (your SMTP relay) does not match the From address domain, and there is no DKIM signature aligned to your domain. Gmail flags the mismatch with 'via'. The fix is DKIM signing with a key published on your own domain so the signing domain and the From domain match. SendMailAs sets up domain-aligned DKIM automatically, which removes the label.
No. Gmail's 'Send mail as' needs SMTP credentials to actually send outgoing mail as your address. Receiving/forwarding services (like Cloudflare Email Routing) do not provide outbound SMTP, so you pair them with an SMTP relay - or use a service like SendMailAs that provides the sending credentials for you.
No. Send Mail As works on a free personal Gmail account. What you need is a correctly configured SMTP relay and aligned SPF/DKIM/DMARC records - not a Workspace subscription. Workspace does not fix these errors; correct authentication does.
Send as your domain, no errors
SendMailAs configures the SMTP relay and aligned SPF, DKIM, and DMARC so Gmail sends as your domain cleanly - inbox, not spam, no "via". First domain free.