Well it's not ELLO if that's what you're using - you've written that a few times. Don't reinvent the wheel. Use an existing library like:
https://www.arduino.cc/reference/en/libraries/emailsender/
I think some reinvention of the wheel may be required as of 2022, because (AFAIK), GMail now refuses to send unsecured emails, and I don't think the various iterations of these Arduino email senders actually use secure SSL (I could be wrong but that's my understanding). A year or so back I was sending GMail with one of these programs, however to do so required going into the GMail settings and disabling secure emails. The problem at the time was a short time later GMail would automatically switch the settings back to secure. Now that setting has been removed from the settings altogether.
Using the latest version (3.0.11) in your link, the program gets hung up after getting the SMTP status code 235 (Authentication successful response), and then returns an error SMTP AUTH error (535-5.7.8 Username and Password not accepted. This error is commonly associated with not having the server setup for less secure apps.
I think this SMTP flow diagram shows how things were at some point, but I believe it's probably out of date as of 2023, I don't even see code 235. Or the mail server I'm connecting to uses an older protocol (perhaps 235 responses instead of 250)?

Any SMTP experts out there?