Outgoing emails for new user registrations would not send from Wordpress. What I'm speaking of is users are not receiving emails when they Register via the Wordpress Register Form. I am receiving emails myself from Wordpress on Tutanota but users weren't receiving the email. When I received a new email from Wordpress, it would be addressed from Me (firstname.lastname@example.org). There was also a little caution symbol that stated the technical sender is different from the email address in the 'From' field. So from a bit of experience with managing enterprise mail servers in the past, I assume this issue is due to the email getting blocked by a Spam Filter of some sort since Wordpress (via PHP Mail function) is just using a generic email by default straight from my server. It's worth noting that tutanota.de is my secure email provider.
So I figured I couldn't use my default PHP Mail function and instead installed a Wordpress SMTP plugin in order to route my emails through Gmail's server. I was refraining from installing the plugin since I try to keep my Wordpress installs as light as possible but went ahead and bit the bullet. I filled out the plugin settings but a test email failed.
Here is the result from my log (some content purposely truncated):
SMTP -> get_lines(): $str is "534-5.7.9 Application-specific password required. Learn more at"
SMTP -> get_lines(): $data is "534-5.7.9 Application-specific password required. Learn more at"
SMTP -> get_lines(): $str is "534 5.7.9 https://support.google.com/mail/?p=InvalidSecondFactor.5 - gsmtp"
Immediately I could tell that I had an issue with the 2-factor authentication that was enabled on my account. So to resolve the issue I did a bit of research and found that you must generate an App Password in order to basically generate a backdoor full access password into your account. This password must be kept very secure as it kind of defeats the purpose of having 2-factor authentication enabled in the first place if it leaks.
I changed my Wordpress SMTP addon password to my new "App Password" I generated. Afterwards, I performed another email test which passed this time with flying colors. I then created a new account for my website using a gmail.com email address which I previous never received any emails. Within seconds, I received the email to click the link and set my Wordpress password. I noticed a few things I need to tweak (sender name, etc.) but at least it is working properly now.