On imports of message-archives from another host, the combined size of the ".mbox" file and the "attachments" directory is initially limited to a maximum of 1GB.
There is no charge for importing the initial 1 GB.
For new-list imports of more than 1GB there is a one-time fee of USD 10.00 per GB or part thereof (not including the initial 1GB).
When your archive (larger than 1GB) has been imported, an invoice will be available in your account.
After that there are no limits or fees applied to the size of the archive as it increases over time.