Why the at sign
Tomlinson needed a character that did not appear in names. The @ was on the keyboard and meant “at” (user at host). He did not remember the content of the first message; it was probably something like “QWERTYUIOP”.
Spam
The first mass advertising message was sent in 1978 by Gary Thuerk of DEC to about 400 people. Today spam makes up a large share of all email, and filters work thanks to machine learning.
Email on your own domain
An address such as [email protected] looks more trustworthy than one at a free service. For reliable delivery you need SPF, DKIM and DMARC records, or messages end up in spam. I can help set it up.