As of Lyris version 3.0, Lyris can send one-off messages directly in memory, without using the outgoing mail queue.
The benefit of this approach is that one-off messages are sent immediately, which is good, because these effect the user perception of how quick your Lyris server is.
Also, by sending one-off messages in memory, rather than through the outgoing mail queue, you save the extra work of writing and reading the message to disk, and you also slow the growth rate of the
outgoing mail database table.
The downside to In-Memory processing is that you cannot view sent one-off messages in the "outgoing mail" web page of the Lyris web interface. If you disable the In-Memory processing feature, you
will be able to view sent one-off messages in the "outgoing mail" web page of the Lyris web interface.
We recommend that most sites implement the In-Memory process, unless you absolutely need the extra information for troubleshooting purposes.