IMAP authentication uses a mailing systems IMAP protocol to check for users accounts.

It allows you to fully integrate your ProtonMail account with most clients that support IMAP and SMTP protocols, with official support for Microsoft Outlook, Mozilla Thunderbird and Apple Mail.

Most computers and mobile devices have a mail program built into their operating system. Use this parameter to specify return path upon mail delivery failure. The receivers specified in bcc will get the mail, but are excluded from the headers. As string with additional headers to be set on the mail cc. The mail body, see imap_mail_compose() additional_headers. It contrasts with email service accessible through a specialised email client software.