Because you are spamming a large number of email addresses (9,236) with the exact same content, your host may have a limit on how many emails can be sent within a set period of time. Your host may set limits on how many emails you can send in one-shot and to how many email addresses you can add to your BCC/CC lines. This may also be limited be length within your hosts mail servers configuration by how long a "TO" address can be.
That is extremely common, and is the reason why there are proper email marketing programs and services.
have a look at https://www.phplist.com/
Services and programs such as phpList Manager, MailChimp are where UBB.thread's option to export email address become extremely useful.
Its recommended that prior to spamming your webhosts mail server, you contact them to learn about your account's limitations, such as, total sent emails per period of time.
Anyway, does the Admin Log entry confirm the emails were definitely sent?
The admin log entry is a confirmation of what was passed through UBB.thread's mailer script to your mail server. Your mail server may have its own log of what it received (from you) vs what it actually sent (away from the server).