Unable to use email identities

Hello,

apologies if there’s already a topic/help article for this, but I haven’t been able to find it.

In my Disroot webmail, there’s a section called “Identities” which allows me to set up custom identities and
allows me to enter (apparently) an arbitrary email address.

When setting up one random identity for testing and attempting to send an email from that identity using Disroot webmail, following error appears:

Failed to add recipient “recipient_email_address”: 553 5.7.1 <disroot_custom_identity_email_address>: Sender address rejected: not owned by user username

When attempting to send an email to that same address from another email provider, I get Undelivered Mail Returned to Sender, with details:

<[disroot_custom_identity_email_address]>: host (disroot_host_address) said: 554 5.7.1 <[disroot_custom_identity_email_address]>: Relay access denied (in reply to RCPT TO command)
Action: failed
Status: 5.7.1
Remote-MTA: dns; (disroot_host_address)
Diagnostic-Code: smtp; 554 5.7.1 <[disroot_custom_identity_email_address]>: Relay access denied

I might be misunderstanding what “Identities” are used for / offer to the user, any clarification and maybe possible solution to this would be appreciated.

Thank you in advance.