Force SMTP to send outside server

Molive · Feb 12, 2025

Hello,

I would like to use an mail account in my ISPconfig to only send mails from an external application while emails for this domain are managed by another server (MX record).

Problem: when I send an email to a recipient in the same domain, Postfix tries to deliver it locally.

How can I do to force it to send emails outside my server, even if the recipient domain is declared in ISPconfig?

Thanks in advance.

Molive

till · Feb 12, 2025

Do not declare the recipient domain in ISPConfig. If a domain is a local email domain on your server, the server will catch all emails for that domain.

Molive · Feb 12, 2025

Thank you but I think my description wasn't clear enough.

Maybe with an example of what I want to do :
- mydomain.com emails are managed by Microsoft.
- But I want to configure My ISPconfig to send emails with [email protected] as sender for an application which cannot use Microsoft SMTP (because of their MFA)
- It works when I send emails to recipients outside mydomain.com, but if I send to [email protected] ISPconfig server thinks it has to deliver it locally and obviously doesn't find the corresponding account

I understand it's a normal behavior, but is there a way to bypass it?

till · Feb 13, 2025

I understand your first question, and my suggestion was based on that. As I mentioned, when you configure a domain as local domain, then postfix will catch all emails for it. Therefore my suggestion to remove it from local domains and use another domain for sending and smtp authentication. Here is a more in-depth answer from ChatGPT:

When Postfix thinks a domain is “local” (because it appears in one of your local domain lists like mydestination, virtual_alias_domains, or virtual_mailbox_domains), it will try to deliver all addresses for that domain locally. There isn’t a built‐in setting to say “only accept [email protected] locally and send all other example.com mail out via DNS MX.”

The common solution is to remove the domain from your local domain lists and then use an explicit mapping for the single address you want to handle locally. For example:

Remove example.com from your local domains:
Make sure that example.com is not in your mydestination, virtual_alias_domains, or virtual_mailbox_domains settings. This tells Postfix that, by default, it should not consider example.com as local.

Set up a virtual alias for the local address:
In your virtual_alias_maps, add an entry such as:

[email protected] localuser

This mapping ensures that mail to [email protected] is delivered locally (to the local mailbox or further processed as you need), while mail sent to any other address at example.com will be handled as non-local mail and delivered using a DNS MX lookup.

Optional – Use transport maps:
If you need more granular control (for example, to route other addresses to a specific remote host rather than doing a generic MX lookup), you can use transport_maps to specify exactly where mail should be sent.

Summary

Problem: Postfix is trying to deliver all mail for example.com locally because it thinks the entire domain is local.

Solution: Remove example.com from your local domains and explicitly map [email protected] for local delivery. All other addresses will be delivered externally (via MX lookup or as specified in a transport map).

This way, Postfix will only “catch” the one mailbox you want locally while letting mail for any other address in the domain go out to the correct external server.

Feel free to ask if you need more details on any of these steps!
Click to expand...

When you add a email domain in ISPConfig, it gets added to virtual_mailbox_domains, therefore postfix must catch the emails. Solution 2 does not apply to your problem, as you can't send then.
Solution 3 (via chatgpt):

You can tell Postfix to route mail for a domain (or even specific addresses) to a different destination using transport maps. In your case, you want:

[email protected] to be delivered locally.

Any other address at example.com to be delivered to the external mail server.

Here’s how to set that up:

1. Remove example.com from Local Domain Lists
First, ensure that example.com is not listed in your local domains (i.e. not in mydestination, virtual_alias_domains, or virtual_mailbox_domains). This prevents Postfix from trying to handle the entire domain locally.

2. Create/Edit the Transport Map File
Create (or edit) a file—commonly located at /etc/postfix/transport—with the following contents:

[email protected] local:
example.com smtp:[remote.mailserver.example.com]

[email protected] local:
This line tells Postfix to use the local delivery method for that specific address.

example.com smtp:[remote.mailserver.example.com]
This line instructs Postfix that all other mail addressed to example.com should be sent using SMTP to the remote mail server.
Note: The square brackets around the host name (e.g., [remote.mailserver.example.com]) bypass MX lookup and force delivery to that host directly. Remove the brackets if you prefer Postfix to do an MX lookup.

3. Postmap the File
After saving your transport file, build the hash database by running:

sudo postmap /etc/postfix/transport

This will create /etc/postfix/transport.db.

4. Configure Postfix to Use the Transport Map
Edit your main.cf (usually in /etc/postfix/main.cf) and add or modify the transport_maps parameter:

transport_maps = hash:/etc/postfix/transport

5. Reload Postfix
Apply the changes by reloading Postfix:

sudo systemctl reload postfix

How It Works

When an email is sent to [email protected], Postfix matches the specific entry and delivers it locally.

For any other address at example.com, Postfix uses the more general mapping and forwards the mail to the remote server (remote.mailserver.example.com).

This setup allows you to precisely control which addresses are handled locally and which are relayed externally.
Click to expand...

It is not a solution for your case as you won't be able to send them either. And just a general note, instead of editing transports file, you would use transports settings in ISPConfig. But as mentioned, this suggestion does not solve your problem as well.

Therefore, remove that domain and send trough another domain, e.g. a subdomain. So instead of adding e.g. yourdomain.tld in postfix, add smtp.yourdomain.tld in postfix and use that for sending. This way, you can send to yourdomain.tld as the email domain of this server is smtp.yourdomain.tld.

Molive · Feb 13, 2025

Thank you Till,
I'll use a subddomain, it's pretty much simpler.
Thank you for your suggestions

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Force SMTP to send outside server

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Force SMTP to send outside server

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