Well, it depends what you call spam, how well known your server is (are your email addresses spread far and wide on the web or only known to a couple of people) but a lot of spam is automated and algorithmic, so most servers will be showered with speculative mail addressed to likely mailboxes - which your server still has to process, if only to bounce the message; if you have antispam measures, your server can just drop the connection when it detects a spammy sender (e.g. from an address on a black or greylist). I’m not currently running any mail servers, but a few years back when I did, I used to get about 80% spam incoming.