We’ve also seen reports of this happening with Apple Mail on Yosemite when SpamSieve is not installed. Also, you indicated via e-mail that it was Mail (not SpamSieve) that was showing the high RAM use and that the problem doesn’t happen when Mail is receiving messages (which is when SpamSieve is doing stuff) but rather when Mail is syncing its mailboxes with the server. So I think it’s likely that the problem is caused by a bug in Mail or the OS and is not related to SpamSieve.
If anyone else is encountering this problem, please answer the following:
Which type(s) of mail account are you using: POP, IMAP, Exchange?
Does it help if you uncheck the SpamSieve rule in Mail’s preferences?
Does it help if you leave the SpamSieve rule active but remove the SpamSieve.mailbundle plug-in?
Does it help if you quit other apps, e.g. iTunes?
When Mail is using lots of memory, you could run this command in Terminal to create a file on your desktop that records what Mail is using the memory for:
heap -guessNonObjects Mail > ~/Desktop/MailHeap.log