There's a few filters that could result in that rejection description (assuming the default rejection messages have not been modified from the SpamFilter.ini file).

The most common ones are:

Email blocked due to the Bayesian filter

Blacklisted keywords (in the "Blacklist / whitelist Tab")

Blacklisted attachments (in the "Blacklist / whitelist Tab")

Exceeded "Max number of spaces in subject line" (in the "Filter Settings" tab)

An empty email containing only the attachment specified in the "BlockBlankEmailsWithAttachments" setting in the SpamFilter.ini file

To be certain of which one was the responsible filter, you can check either SpamFilter's activity logfile (which will log the filter used to block emails), or, if you have access to it, look at the "X-Rejection-Reason" header in the blocked email (it will indicate the filter that blocked it).

Please note that sometimes spammers will encode the subject lines in the email's source so that the rendered text will be different than the encoded one. In these cases you will need to view the actual email's source to see what the real subject header was.

If that is not the case, please contact us via email at support at logsat.com so we can ask you to provide us with the SpamFilter's activity logfile for a day this happened, the contents of your SpamFilter\Domains directory, and some of the original emails in question (please note that we'll need the original, unmodified email source - some email clients, notably Microsoft Outlook, completely change the email format making it impossible without registry tweaks to have access to the actual source).

If the emails are in the quarantine, SpamFilter did indeed reject them, either thanks to your keyword or due to another filter.

Please note that, if you wish, you can configure most filter so that emails that are blocked by them are permanently rejected and not stored in the quarantine.