A spam filter is the automated system mailbox providers use to decide whether an incoming email belongs in the inbox, the spam folder, or nowhere at all. Filters weigh many signals: the sender's reputation, authentication, the content and formatting of the message, links and attachments, and how previous recipients treated similar mail.
For cold outreach, spam filters are the gatekeeper between your message and the hiring manager's attention. Triggers that raise spam risk include spammy phrasing and excessive links, sending to many invalid addresses (high bounce rate), blasting high volume from a new domain, and attaching files on a first contact.
The way to stay out of the spam folder is to behave like a real person writing a real message: send to verified addresses, keep volume reasonable, write naturally, link instead of attaching, and maintain a healthy sender reputation. Genuine personalized outreach rarely looks like spam — because it isn't.