The wording is a bit ambiguous — “server-side email processing” refers to processing emails which are stored with an email provider, e.g. Gmail. IMAP is used to

The wording is a bit ambiguous — “server-side email processing” refers to processing emails which are stored with an email provider, e.g. Gmail. IMAP is used to interact with the email providers because it is a mail retrieval protocol, and is widely used. If we were doing “server-side email processing” of emails as they were sent, SMTP would have fit in.