SendMessageNL is often the best option for a fast bulk import and email sending but users should be aware of some known limitations

• Import process time leads to a delay for immediate sending requests. Delay duration depends on the size of the file to be imported.
• Imported files should be periodically removed from admin console user space to avoid to be charged of exceeding user space
• If you specify a start date/time that occurs in the past then your request is enqueued for immediate sending.
• In some cases message scheduling could fail (e.g. when another sending is planned at specified date and time, in this case you can use GetNewsletterQueues to retrieve available time slots before calling SendMessageNL). Scheduling failure is notified by email to the technical contact that is registered on MailUp admin console. An email message from support@mailup.it will then notify user when operation cannot be completed, also reporting an error code that is detailed here
• if the file to be imported contains more than one milion of recipients please contact us

Some of the restrictions listed above will be removed in next MailUp releases, please periodically check this page or MailUp release notes.