The plugin stopped dead

This plugin was quite complex to set up but I finally got it working. Using SSL on SMTP Port 465 with a G Suite account. Set it up identically on a second domain for development purposes using the same G Suite account & email & OAUTH codes and it too worked great. Then as I got down to testing emails sent I decided I wanted to use a ‘noreply’ email on the same domain for initial emails anyway, to dodge spammers. G Suite support helped me set that up and it worked fine: I could send emails from noreply but any email sent to it was bounced back, exactly as it should work. But when I changed the From email address to the noreply in the plugin’s setup, it wouldn’t work. So I thought maybe it had to match with the OAuth Email Address setting and changed that too but it didn’t work in that case either. So I put it back the way it was with the original email address in both those fields, but it STILL didn’t work! And the identical setup on the other domain, which I hadn’t touched, also didn’t work anymore.

So I called G Suite support thinking maybe I had triggered some kind of flag in Gmail somehow maybe by changing the email address that made it stop accepting my emails, but the support guy said there wasn’t any such flag. He gave me a URL where I could try disabling Gmail’s ReCaptcha nuisance, but that didn’t make any difference either. I can still send and receive emails through the Gmail account on my old Blackberry so it doesn’t seem to be a Gmail problem really, and the Support guy eventually told me we were outside the scope of his job and that I should get a WordPress developer to work on it.

This is my ‘Server Info’ from the plugin’s option for that:
OS: Linux us-imm-web136.main-hosting.eu 3.10.0-962.3.2.lve1.5.25.6.el7.x86_64 #1 SMP Thu Apr 18 06:40:26 EDT 2019 x86_64
PHP version: 7.3.5
WordPress version: 5.2.2
WordPress multisite: No
openssl: Available
allow_url_fopen: Enabled
stream_socket_client: Available
fsockopen: Available
cURL: Available
cURL Version: 7.62.0, OpenSSL/1.0.2k

This is what happens with Test Email:
2019-08-10 23:48:58 Connection: opening to ssl://smtp.gmail.com:465, timeout=300, options=array ()
2019-08-10 23:48:58 Connection: opened
2019-08-10 23:48:58 SMTP -> get_lines(): $data is “”
2019-08-10 23:48:58 SMTP -> get_lines(): $str is “220 smtp.gmail.com ESMTP b64sm20152769ywe.43 – gsmtp”
2019-08-10 23:48:58 SERVER -> CLIENT: 220 smtp.gmail.com ESMTP b64sm20152769ywe.43 – gsmtp
2019-08-10 23:48:58 CLIENT -> SERVER: EHLO nukewar.info
2019-08-10 23:48:58 SMTP -> get_lines(): $data is “”
2019-08-10 23:48:58 SMTP -> get_lines(): $str is “250-smtp.gmail.com at your service, [2a02:4780:1:3::1c]”
2019-08-10 23:48:58 SMTP -> get_lines(): $data is “250-smtp.gmail.com at your service, [2a02:4780:1:3::1c]”
2019-08-10 23:48:58 SMTP -> get_lines(): $str is “250-SIZE 35882577”
2019-08-10 23:48:58 SMTP -> get_lines(): $data is “250-smtp.gmail.com at your service, [2a02:4780:1:3::1c]250-SIZE 35882577”
2019-08-10 23:48:58 SMTP -> get_lines(): $str is “250-8BITMIME”
2019-08-10 23:48:58 SMTP -> get_lines(): $data is “250-smtp.gmail.com at your service, [2a02:4780:1:3::1c]250-SIZE 35882577250-8BITMIME”
2019-08-10 23:48:58 SMTP -> get_lines(): $str is “250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH”
2019-08-10 23:48:58 SMTP -> get_lines(): $data is “250-smtp.gmail.com at your service, [2a02:4780:1:3::1c]250-SIZE 35882577250-8BITMIME250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH”
2019-08-10 23:48:58 SMTP -> get_lines(): $str is “250-ENHANCEDSTATUSCODES”
2019-08-10 23:48:58 SMTP -> get_lines(): $data is “250-smtp.gmail.com at your service, [2a02:4780:1:3::1c]250-SIZE 35882577250-8BITMIME250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH250-ENHANCEDSTATUSCODES”
2019-08-10 23:48:58 SMTP -> get_lines(): $str is “250-PIPELINING”
2019-08-10 23:48:58 SMTP -> get_lines(): $data is “250-smtp.gmail.com at your service, [2a02:4780:1:3::1c]250-SIZE 35882577250-8BITMIME250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH250-ENHANCEDSTATUSCODES250-PIPELINING”
2019-08-10 23:48:58 SMTP -> get_lines(): $str is “250-CHUNKING”
2019-08-10 23:48:58 SMTP -> get_lines(): $data is “250-smtp.gmail.com at your service, [2a02:4780:1:3::1c]250-SIZE 35882577250-8BITMIME250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH250-ENHANCEDSTATUSCODES250-PIPELINING250-CHUNKING”
2019-08-10 23:48:58 SMTP -> get_lines(): $str is “250 SMTPUTF8”
2019-08-10 23:48:58 SERVER -> CLIENT: 250-smtp.gmail.com at your service, [2a02:4780:1:3::1c]250-SIZE 35882577250-8BITMIME250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH250-ENHANCEDSTATUSCODES250-PIPELINING250-CHUNKING250 SMTPUTF8
2019-08-10 23:48:58 Auth method requested: XOAUTH2
2019-08-10 23:48:58 Auth methods available on the server: LOGIN,PLAIN,XOAUTH2,PLAIN-CLIENTTOKEN,OAUTHBEARER,XOAUTH
2019-08-10 23:48:59 CLIENT -> SERVER: AUTH XOAUTH2 dXNlcj1yYWRpYXRpb25jb25zdWx0aW5nQG51a2V3YXIuaW5mbwFhdXRoPUJlYXJlciABAQ==
2019-08-10 23:48:59 SMTP -> get_lines(): $data is “”
2019-08-10 23:48:59 SMTP -> get_lines(): $str is “334 eyJzdGF0dXMiOiI0MDAiLCJzY2hlbWVzIjoiQmVhcmVyIiwic2NvcGUiOiJodHRwczovL21haWwuZ29vZ2xlLmNvbS8ifQ==”
2019-08-10 23:48:59 SERVER -> CLIENT: 334 eyJzdGF0dXMiOiI0MDAiLCJzY2hlbWVzIjoiQmVhcmVyIiwic2NvcGUiOiJodHRwczovL21haWwuZ29vZ2xlLmNvbS8ifQ==
2019-08-10 23:48:59 SMTP ERROR: AUTH command failed: 334 eyJzdGF0dXMiOiI0MDAiLCJzY2hlbWVzIjoiQmVhcmVyIiwic2NvcGUiOiJodHRwczovL21haWwuZ29vZ2xlLmNvbS8ifQ==
SMTP Error: Could not authenticate.
2019-08-10 23:48:59 CLIENT -> SERVER: QUIT
2019-08-10 23:48:59 SMTP -> get_lines(): $data is “”
2019-08-10 23:48:59 SMTP -> get_lines(): $str is “535-5.7.8 Username and Password not accepted. Learn more at”
2019-08-10 23:48:59 SMTP -> get_lines(): $data is “535-5.7.8 Username and Password not accepted. Learn more at”
2019-08-10 23:48:59 SMTP -> get_lines(): $str is “535 5.7.8 https://support.google.com/mail/?p=BadCredentials b64sm20152769ywe.43 – gsmtp”
2019-08-10 23:48:59 SERVER -> CLIENT: 535-5.7.8 Username and Password not accepted. Learn more at535 5.7.8 https://support.google.com/mail/?p=BadCredentials b64sm20152769ywe.43 – gsmtp
2019-08-10 23:48:59 SMTP ERROR: QUIT command failed: 535-5.7.8 Username and Password not accepted. Learn more at535 5.7.8 https://support.google.com/mail/?p=BadCredentials b64sm20152769ywe.43 – gsmtp
2019-08-10 23:48:59 Connection: closed
SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting