What type of hardware are you using: VPS bought online
What YunoHost version are you running: 12.1.39
How are you able to access your server: SSH
https://belin.land is connected to my YunoHost server (no connected apps). Nevertheless, every email I send to tommi@belin.land is rejected… Why could this be?
Hey @tommi 
The error message is quite explicit:
550 5.1.1 <tommi@belin.land>: Recipient address rejected: User unknown in virtual mailbox table
This means your server does receive the email (DNS/MX are working fine), but Postfix rejects it because no mailbox exists for tommi@belin.land in the LDAP directory.
In YunoHost, simply adding a domain to your server doesn’t automatically create any email address on it. Each user gets an email address in the format username@domain.tld upon creation, and additional email aliases can be configured for each user.
So you have two options:
Add tommi@belin.land as an email alias to an existing YunoHost user: go to Webadmin → Users → [your user] → Manage email aliases, and add tommi@belin.land.
Create a new YunoHost user with belin.land as the primary domain, if you’d rather have a dedicated account.
Either way, once the address is registered in YunoHost’s LDAP, Postfix will accept incoming mail for it.
Hello,
Moreover, your IPv6 points to a different host, or your Postfix SMTP server is not configured to listen to your IPv6 interface.
nc: connect to belin.land (2a02:c206:2111:134::1) port 25 (tcp) failed: Connection refused
Thank you so much, @djez!
I did not think about creating an alias because IMAP configuration for tommi@belin.land worked in Thunderbird even before creating it… 
Nevertheless, I now created an alias but it looks like the email is still rejected 
About IPv6, instead, it’s Contabo going crazy sometimes… Not too reliable from this point of view. It should be working, now? 
Good news: the alias is working! The error has changed, which means the first issue is resolved.
Now, the new error is different:
553 5.7.1 <surfing@tommi.space>: Sender address rejected: not logged in
This happens because tommi.space is also a domain on your YunoHost server. When Proton Mail delivers an email from surfing@tommi.space to your server, Postfix sees the sender address belongs to a local domain but the connection is not authenticated — so it rejects it as a potential impersonation attempt.
This is caused by the reject_sender_login_mismatch directive in Postfix’s configuration.
You have two options:
Quick test: try sending your test email from an address that is not on a domain hosted by your YunoHost (e.g. a Gmail address 
). That should go through fine and confirm the alias works.
Permanent fix: if you actually need to receive emails sent from @tommi.space via Proton Mail, you can comment out reject_sender_login_mismatch in /etc/postfix/main.cf and reload Postfix (systemctl reload postfix). SPF, DKIM and DMARC already protect against sender spoofing, so the security impact is minimal. Note that YunoHost may restore this line on regen-conf postfix, so keep that in mind.
This exact issue was discussed and solved here: Question Postfix -- 553 5.7.1 ... : Sender address rejected: not logged in
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