Individual messages stored in .eml files can be placed in a folder named 'pickup/' inside of an account's directory. These files will routinely be processed and injected into the MTA. This is the slowest injection option and should be used sparingly. For more information please see this related article: How to use the account pickup directory
When injecting messages into the HMS, the management and number of connections matters. The slowest method of SMTP injection would be to connect to the MTA, inject a message, disconnect, and repeat. It's best to keep connections up while injecting multiple messages due to the overhead of opening and closing those connections. Additionally, the MTA supports multiple connections injecting messages over SMTP at the same time. Therefore, the more efficient use of SMTP injection is by opening and maintaining somewhere in the range of 5-30 connections, keeping them open, and injecting many messages across each connection.
Hurricane Assembly Server
The most efficient method for injecting messages into an MTA is by using our Hurricane Assembly Server (HAS). The HAS generates large batches of messages by processing XML configuration files which instruct the HAS on how to combine sets of message templates, data fields, and addresses. The HAS then efficiently injects the resulting batches of messages into the HMS. Click here for more information about our Hurricane Assembly Server product