I can think of 2 relatively easy methods to send an anonymous email. Be careful to not abuse the first method, otherwise, the service provider might decide to restrict or block global access to the service permanently. Use prudence and good judgement when sending anonymous emails. In most jurisdictions, it is a serious crime to send emails that threaten, harass, or contain illegal content. It is easy to inadvertently leave tell-tale clues in the Subject line or message body that could give away your identity. A combination of unique elements such as type of language, grammar, font, etc. have the potential to trace back to you.
Configure Tor browser ▸ Privacy and Security Settings ▸ Tor Network Settings to the highest level. In Tor browser ▸ Network Settings ▸ Enter custom bridges. Consider using the most obfuscated (most secure) type of custom bridge address, obfs4. (The only other type of bridge address that might be more secure is a private obfuscated bridge address that a trusted friend gives to you, but not to the public, after your friend sets up a private Tor bridge relay.)
Use Tor browser to connect to StartMail. Open a free 7-day trial account. Your name, phone number, email address, or other personally identifying information are not required to open the account. Configure your preferred user account settings. Send your anonymous email. The only sender IP address that will appear in the full email header will be the IP address of the StartMail email server. The email header will not include your IP address, your name, your country location, or any other personally identifying information about you.
With Tor browser, connect to Yandex mail. Open an email account. Your name, phone number, email address, or other personally identifying information are not required to open the account. Configure your preferred user account settings. Send your anonymous email. The only sender IP address that will appear in the full email header will be the IP address of the Yandex email server. The email header will not include your IP address, your name, your country location, or any other personally identifying information about you.
The email service providers will be unable to log or record your IP address if you are using the Tor browser with the highest Privacy and Security Settings and an obfuscated bridge address.
In contrast to the StartMail free 7-day trial account, a personal paid subscription to StartMail email service does not provide 100% anonymity, although, to the best of my knowledge, it does provide the highest level of anonymity and privacy from a commercial email service provider that includes a built-in option to send and receive PGP encrypted emails. Additionally, StartMail headquarters are in the Netherlands, beyond the legal jurisdiction of any foreign governments. See StartMail Policies, Technology, and Privacy statements for a better understanding of the practices and methods that StartMail uses to protect a user's personal information, privacy, and anonymity, including guaranteed no tracking or advertising and regular automatic deletion of some of the users' personal information within hours after the session ends.