• Server Level Filters – hosting companies often have filters set server wide which lower level tech support staff are sometimes not even aware of, and may even deny their existence.
• Hosting Filters – a hosting account may have the option to enable and configure an anti-spam script such as SpamAssassin.
• Antivirus – The antivirus software on an individual user’s computer may have its own anti-spam features and thresholds as default settings.
• Email Client – The actual software client such as Outlook or Thunderbird may also have features for filtering and handling spam.

I prefer that all messages reach my email client – spam or otherwise, and use that software’s filtering to sort things out. One trick is to take advantage of subject line modification that may be available at each level. For example: on the hosting level add something to indicate it’s being tagged there like [*Ho*] to the subject line. Additionally the antivirus software may allow tagging of the subject line for example: [*Av*]. You can then set a filter in the email client to sort email with those subject lines automatically to the spam or trash folder. Each of these clues will give an indication of where the “offense” is taking place. Of course, if it arrives with both tags in the subject line, then you know it’s a doozy.