calibre-smtp(1) on Linux
The text 736b6ca3554e, as Debian 11 (Bullseye) shipped it.
calibre-smtp Send mail using the SMTP protocol. calibre-smtp has two modes of operation. In the compose mode you specify from to and text and these are used to build and send an email message. In the filter mode, calibre-smtp reads a complete email message from STDIN and sends it. text is the body of the email message. If text is not specified, a complete email message is read from STDIN. from is the email address of the sender and to is the email address of the recipient. When a complete email is read from STDIN, from and to are only used in the SMTP negotiation, the message headers are not modified. Whenever you pass arguments to calibre-smtp that have spaces in them, enclose the arguments in quotation marks. For example: "/some path/with spaces"