How do you code and send an email?

Question

I do not mean how to mailto:[e-mail] in a website.
I mean put HTML into your email and send it.
I know how to put code into your email by making a text/HTML document and coding it like a webpage but I just don't know how to send it.
Please Help.
Thanks!

Answer 1

Here's how you'd send an HTML email by hand, using telnet (or PuTTY, netcat, ...)
https://www.walkernews.net/2011/01/06/uses-telnet-and-smtp-commands-to-send-email-with-subject-and-html-content/

Following this pattern, you can then:
~ write a program to construct appropriate answers to a mailserver...
~ or at least start learning why mailsystem functions in various programming languages require you to do things in a particular order

There may be better examples out there (including server responses); this was just the first with HTML.

[meta, realtime blackholes]
Be careful with noise / getting the client right to avoid getting RBL'd
https://mxtoolbox.com/problem/blacklist/dns-realtime-blackhole-list
...or filtered by some algorithm. Probably not a concern if you test with your own stuff.

Answer 2

Here's a really nice screenshot of a full SMTP exchange, plus the SWAKS testing tool for mailservers:
https://debian-administration.org/article/633/Testing_SMTP_servers_with_SWAKS

Note: Not everyone enables HTML. You often want to also send a text part, even attachments.
...include "MIME" "base64" and "multipart" in searches once you want to do these things.