Short answer: Samsung Email App does not meet Gmail Security Requirements
Solution: Disable "Less secure app access" in Google account for continuously using the same email client
Recently I have not been able to send mails via Samsung Mail on my all Andoid phones but iPhone works like a charm. Samsung Mail is up-to-date; thus no update is needed. In my Gmail, I received a Critical security alert message regarding the "Sign-in attempt was blocked". Obviously, it was the main reason why I cannot send mails.
Clicking the given "Check activity" button in the mail will bring me to another message. I thought answering "Yes, it was me" could help for unblocking SMTP from my phones. But it doesn't. I need to physically go in my account and then turn off "Less secure app access" security in order for me to use Samsung Email client to send mails. It is clear that Google strengthens its security while Samsung Email does NOT meet Google security. It also explains to me why all Samsung products now keep asking permission to access my Google data which I usually ignore and disregard.
On the other hand, I do receive Critical security alert message about "Sign-in attempt was blocked" whenever I used iPhone to send mails. But the email was successfully passed through Google SMTP authentication and being sent out. Does iPhone email client really meet Google security requirements? If so, why do I receive an alert? Or is it because the alert will be sent whenever all non-Google apps access Google data?
Less secure app blocked
Google blocked the app you were trying to use because it doesn't meet our security standards.
Some apps and devices use less secure sign-in technology, which makes your account more vulnerable. You can turn off access for these apps, which we recommend, or turn on access if you want to use them despite the risks. Google will automatically turn this setting OFF if it's not being used.
Learn more
Enable or disable "Less secure app access":
https://myaccount.google.com/lesssecureapps