FTP based integrations
You can request your MailUp representative for a FTP Batch Process for massive data transfer using files CSV files. This feature can be used for very large amounts of recipients (this kind of process has been successfully tested with 4 millions of recipients in a single file).
You can run MailUpImport SOAP API for an immediate import of recipients. In particular, you can use StartImportProcesses method, optionally combined with NewImportProcess if the amount of recipient is more than 5000. These methods also allow a batch import as "pending", and in this case, the confirmation email has to be manually sent using the admin console account. SOAP API is not bidirectional, so you cannot use it to export recipients from MailUp to your application.
SOAP API + FTP
You can get the best of FTP and SOAP API by using SendMessageNL API method, which performs scheduled sending of a message to a set of recipients that are included in the FTP import file previously processed (message ID and filename are passed as method parameter). In this case you have to transfer the file to MailUp before calling the method. Export of subscribed and unsubscribed recipients can be done with an FTP based integration.
Rest API allows immediate and bidirectional import and export of recipients. Due to restriction on HTTP channel, no you cannot pass more than 7MB input files can be uploaded with as a payload of a single import. For the export of recipients the paging feature is recommended to keep control on maximum size of transferred data. call. To overcome this limit, you may either split your list into multiple calls or provide a large file to be uploaded using FTP (the latter is the reccomended option in case of very large data set). The REST API is the only MailUp solution that allows removal of a single recipient from a group.