As Android, Google and Avast updates that's many moving parts that any one could be the cause. Sorry, no, it does not tell us which or where it is. Only that your mail app and the combination of Avast etc does not work.
As on your PC you reduce the count of things until it works or you are down to one thing.
Samsung S5 Active, AT&T Android version 5.0
As of March 5th, I cannot send e-mails from my g-mail account. I have been using it for over a year with no problems. I did not change any of the settings. I can send e-mail from both my Yahoo & Hotmail accounts. I can read e-mails from Gmail, no problem, so I am signed in.
I checked the outgoing server settings AFTER I started having the problems. It had the field "Sign In Required" checked (as it does with my other e-mail accounts) but I unchecked it, to see if that would help. It didn't, I still couldn't send e-mail. Then, when I switched it BACK to sign in required & tried to sign back in, I get the error message "Unable to set up account - Authentication failed."
I also get an e-mail from Google: Sign-in attempt prevented
Someone just tried to sign in to your Google Account email@example.com from an app that doesn't meet modern security standards.
We strongly recommend that you use a secure app, like Gmail, to access your account. All apps made by Google meet these security standards. Using a less secure app, on the other hand, could leave your account vulnerable. Learn more.
Google stopped this sign-in attempt, but you should review your recently used devices:
Why, all of a sudden does Google think that my device is unsafe?
I am running Avast Mobile Security, but have been since I first got the phone & have not changed any settings. I ran a virus check & it found nothing. I downloaded Malwarebytes Ant-Malware, scanned the phone & it found nothing. I then uninstalled the Malwarebyte app.
I have a few downloaded apps (not as many as most people that I know!) but did not load anything immediately before this problem happened. I noticed that some apps ask for a lot of permissions, but don't know why any would somehow change any of the outgoing settings & why just Gmail? If there is a conflict with any of the apps I'm hoping that I'm not the only one & that somehow the bug will get fixed, but it's been 3 weeks now & I still have the same problem.