1Check the mailbox and its password
Each IONOS email address has its own password, separate from your IONOS account login. Sign in to IONOS Webmail with the address and password you plan to use; if that works, SMTP will accept them too.
These settings are for IONOS Mail Basic and Mail Business. If you bought Microsoft 365 through IONOS, use the Microsoft 365 settings instead.
2Create the FormSMTP project
Sign up free and add a project. There’s no IONOS preset in the provider list, so leave it on Custom and type the host and port below.
3Enter the settings
| Provider | Custom |
| Host | smtp.ionos.com (United States), smtp.ionos.co.uk (United Kingdom) |
| Port | 465 with SSL on. If that fails, 587 with SSL off (STARTTLS). |
| Username | Your full email address |
| Password | The password for that email address |
| From email | The same address, or another address on the same domain |
The host can differ by country. Outside the US and UK, open the server details page on your country’s IONOS site and use the host it lists.
Click Test connection, then add your form. See the HTML contact form guide, or compare every provider on the SMTP settings page.
Hourly limits grow with the mailbox’s age
IONOS limits how many emails an address can send per hour based on when it was created:
| Mailbox age | Emails per hour |
|---|---|
| 0–7 days | 50 |
| 8–14 days | 100 |
| 15–30 days | 400 |
| Over 30 days | 500 |
A contact form rarely gets near even the first tier. Keep it in mind if you create a fresh website@yourdomain.com address for the form: it starts at 50 an hour for its first week, which is fine for messages from visitors but not for a launch-day announcement. If you also plan to send newsletters, connect a sending service such as Brevo or Amazon SES for that.
Sender addresses
Since January 29, 2024, IONOS only lets a mailbox send as a different address if it’s on the same domain. Sending as hello@yourdomain.com while signed in as website@yourdomain.com works; sending as an address on another domain doesn’t.
That rule never gets in the way of a contact form. FormSMTP sends from your own address and puts the visitor’s email in the Reply-To header, so the visitor’s address is never the sender. When you hit Reply in IONOS Webmail or your mail app, the answer goes straight to the visitor.
Common errors
- 535 Authentication failed: the password is your IONOS account password rather than the mailbox’s, or the username isn’t the full address. Test the pair in IONOS Webmail.
- 450 Mail send limit exceeded: the mailbox reached its hourly limit for its age. It resets after an hour.
- Sender address rejected: the From address is on a different domain from the mailbox.
- Timeouts: switch to 587 with SSL off, as IONOS suggests when 465 doesn’t connect.
For anything not listed here, look the code up in SMTP error codes explained.
Sources: IONOS mail server details (US) · IONOS mail server details (UK) · Sending limits. Checked November 2, 2026.
FAQ
What is the IONOS SMTP server?
smtp.ionos.com on port 465 with SSL/TLS for the US, and smtp.ionos.co.uk for the UK. Elsewhere, use the host on your country’s IONOS help site.
Why can I only send 50 emails an hour?
The mailbox is less than a week old. IONOS raises the hourly limit as the address ages, up to 500 an hour after 30 days.
Is my IONOS login the SMTP password?
No. Use the password of the email address itself, which you can check by signing in to IONOS Webmail.
Can I send as an address on another domain?
Not through IONOS. Since January 29, 2024, a mailbox can only send as other addresses on its own domain. Use a mailbox on the domain you want to send from.
Why do my messages land in spam?
Usually the From address isn’t on a domain you’ve verified with the provider, or the domain has no SPF and DKIM records. Verify the domain, send from an address on it, and check the Deliverability panel in FormSMTP’s Settings.