A slow throughput can be caused by different elements:
You may have been blacklisted by some Email Providers, you can check this in the broadlogs.
Your email is too big to be processed quickly (high javascript personalization, weighs more than 60kbytes). Note that the recommended maximum size of an email is around 35 kb. You can check the message size, in the preview tab: choose a recipient and once generated the message size will be displayed in the top right corner of the preview tab.
Another common cause of slow delivery speeds is when throttling occurs within the Adobe Campaign MTA. The causes of this sort of throttling are:
Messages pended (quotas met): the declarative MX rules defined in Campaign declare quotas, and these quotas have been met.
Messages pended (dynamic flow control): Campaign MTA has encountered errors when trying to deliver messages for a given ISP and it has decided to slow down (to avoid having too big of an error density and thus facing potential blacklisting).
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