If it happens on gmail, and you use an app to send/receive, it might be because you use a “less secure app” (eg. a depreciated email smartphone app)
Try to enable less secure app access in https://myaccount.google.com
(If you have several gmail account, check if this page is connected to the right account).
In my case it was OFF, I could download the mail (which is weird) but could not send them.