Instant AntiSpam has a max message size limit that when reached will not scan the message (Spam Assassin/Bayesian).
The max size limit is 512K and this limitation size is high enough where spammers will not be sending messages at that size, meaning all messages below 512K will get scanned.
Messages that contain attachments and are below total message size of 512K with get scanned only up to the attachment, so the headers, and message body will get scanned but not the attachment, this is usually very efficient.
When a message is sent that contains up to 512K of content in the message body (no attachments) the content in the message body will get scanned. This could take a while to scan and will consume CPU cycles to process both the SpamAssassin and Bayesian filtering on that size of message.
Here are some recommendations to try.
If one or more of your users need to send a message that is larger then 25K (this could be a dozen + paragraphs) have them type it in a word processor and attach it.
If internal users are in the habit of creating large messages in the body you can add the internal IP address of your network to the by-pass file of Instant AntiSpam (: button next to Spam Engine), this will by-pass internal scanning and allow users to send messages with any amount in the body.
Set the max message size to scan at a lower amount. If you set this to a lower amount keep in mind that, although rare, a spammer could send a message above the max limit and get around spam filtering. We recommend setting it to 256K.
Open the VMS Admin, Security, Spam Engine button, HTML & Other tab and select the Settings File button. Change the following string
Save the file and cycle all the VisNetic MailServer services.