calibre does not ever send mails without attachments. This will be a bug in pbsync. Send them to your normal email address instead to confirm. Most likely it will be because those books have non-english title/authors and the pbsync service cant deal with unicode filenames for attachements. If you run up-to-date clibre it detects mails sent to @pbsync.com and uses ascii filenames for them automatically.
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