This issue is caused by a legacy v5 feature that is still enabled (portal validation) which is not supported in Horizon and needs to be disabled in the database for the error to be resolved.

Error message:

You have requested a non-existent service "portal_validation"
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In the mail processing debug log, will look similar to this

[2025-01-06T11:16:46Z DEBUG] [TicketGatewayProcessor] Creating new ticket

[2025-01-06T11:16:46Z DEBUG] [TicketGatewayProcessor] Found existing person: 1445
[2025-01-06T11:16:46Z DEBUG] User is not confirmed, message is rejected
[2025-01-06T11:16:46Z DEBUG] --> Processor exception: 0 You have requested a non-existent service "portal_validation". 
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The emails will not be pulled into Deskpro and failing with the generic error: Status: ERROR server_error

You will need to access your server on the command line, enter MySQL and run the following against your production database

UPDATE settings SET value = 0 WHERE name = 'core_tickets.email_require_validation';
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This will disable the feature and allow the email account to process the incoming emails as normal.