In an otherwise unmodified Plesk installation, default service plans restrict a given mailbox to a default size of 100MB. With Plesk Premium Email, this maximum size should probably be increased to a more sensible volume of storage.

There are a couple of ways to change the mail quota settings for mailboxes;

• Increase the quota for each mailbox manually (not recommended),
• Increase the quota by customizing the subscription’s limits (not recommended),
• Change the service plan definition (recommended).