Timeout and/or memory owerflow
Using TYPO3 4.5.2 and tt_products 2.6.2 the orders (~35k) could not be sent because of these (or timeout) message:
Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 4548863 bytes) in /srv/www/htdocs/web56/html/t3/typo3/contrib/swiftmailer/classes/Swift/Encoder/QpEncoder.php on line 192
In Accordance with Franz Holzinger it seems to be an error in swiftmailer. Several other EXT (e.g. mm_forum, directmail, sr_feuser_register, powermail) seems to have problems to, but mostly it could be resolved by setting [MAIL][substituteOldMailAPI] = 0
This also helps me in my case.