I just looked again into the log of “Post SMTP”. There appears ONLY the second mail – that is, the one that has no links to the picture thumbnails. The first one, which has the correct links, does not appear in the log.
As a precaution I have increased the following values. Let’s see if it does anything.
php_value max_input_vars 3000
php_value max_execution_time 300
php_value post_max_size 256M
Maybe you can think of something else 😉