When you send a message with an attachment to a mail address with disabled mailbox service and enabled redirect, it will be rejected if the quota of the mailbox is smaller than the size of the attachment.
The logfiles show:
qmail-queue-handlers: handlers_stderr: DATA Mailbox full#015#012DEFER qmail-queue-handlers: DEFER during call 'check-quota' handler
When there is a forwarding mail account, the mail quota is checked, whether the mail account is enabled or not.
Currently, there is no way to disable the quota handler for an individual mail account; this configuration must be set globally:
# /usr/local/psa/admin/bin/mail_handlers_control --list |grep quota | X | 10 | all | check-quota | global | before-queue
The issue will be fixed in a future update for which a new Knowledgebase article will be created. You may subscribe to new articles on Parallels Plesk Panel products if you wish to receive notifications about their creation.
Increase the mail quota and deactivate the mailbox. This will allow larger mail messages to be redirected.