Go to a mail console on that character and withdraw all of its returned items from the mail.
A while back it was changed. Each item that had expired in the exchange and returned to you in the mail reduces you exchange slots by 1 for each item. So if that character has 10 items still in the mail box that were returned from the exchange, then that character can only post 30 items for sale instead of 40. This is intentional and to prevent players from using the mail system as a nearly infinite bank.
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A while back it was changed. Each item that had expired in the exchange and returned to you in the mail reduces you exchange slots by 1 for each item. So if that character has 10 items still in the mail box that were returned from the exchange, then that character can only post 30 items for sale instead of 40. This is intentional and to prevent players from using the mail system as a nearly infinite bank.
Do you have any link showing the current limits for our mailbox ?