Assistant sends email through a Gmail account of yours, using an app password, never your regular password. You need a Google account with 2-step verification turned on.
1. Create the app password. Go to your Google account → Security → turn on 2-step verification if you have not → find App passwords → create a new one (the name does not matter, for example "Assistant"). Google gives you a 16-letter key; copy it. The official help is at support.google.com/accounts/answer/185833.
2. Paste it into Assistant. In Settings → Notification - File, under Email credentials (Gmail): type your Gmail address, paste the app password (with or without spaces, either works), set the sender name your customers will see, and the addresses where you want alerts and reports.
3. Save and test it. Press SAVE SETTINGS and then SEND TEST EMAIL. If you get an authentication error, it is almost always the regular Gmail password being used instead of the 16-letter app password.
The account is yours and the sending rules are Google's: if Gmail limits or blocks sends, that is solved in your Google account, not in Assistant.