In .env file change

“MAIL_ENCRYPTION=starttls” to “MAIL_ENCRYPTION=tls”

In conf/mail.php, line 59 set MAIL_FROM_ADDRESS to be same as in .env file MAIL_USERNAME value

'host' => env('MAIL_HOST', 'your-mail-host'),
  'from' => [
        'address' => env('MAIL_FROM_ADDRESS', 'your-mail@host.com'),
        'name' => env('MAIL_FROM_NAME', 'Your User Name'),
    ],

.env file

...
MAIL_USERNAME=your-mail@host.com
...

• https://stackoverflow.com/questions/62577544/how-do-i-use-starttls-with-swiftmailer-in-php
• https://stackoverflow.com/questions/52765247/expected-response-code-250-but-got-code-554-with-message-554-5-2-0-stor-edr
• https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365