I know this is an old topic, but I have the proposed solution. I have contacted Microsoft about this issue and we figured out the solution for this problem.
Install the following update on the SQL2012 SP1 installation.
http://support.microsoft.com/kb/2894115
When using Windows SMTP server. Get rid of the SMTP limits "Limit Session Size KB" and "Limit number of messages by connection to"
(my SMTP server was running on Windows 2008-R2. )
Read the entire solution below.
http://dba60k.net/index.php/2013/12/23/ssrs-2012-mails-missing-when-using-data-driven-subscriptions-and-bulk-mail/[/url]
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MS-SQL / SSIS / SSRS junkie
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