After providing all the necessary fields, we receive the below issue during Test Connection.

Connection Test Failed! Here is what we got instead:

500

5.3.3 Unrecognized command 'b2Rvb0B0ZW50aHBsYW5ldC5pbg==' [HK2PR02CA0177.apcprd02.prod.outlook.com]

We use Microsoft Office 365 mail server.

Additional Details

SMTP Server : smtp.office365.com

Priority : 1

SMTP Port : 587

Connection Security : TLS (STARTTLS)

Any possible quick work-around to fix?

Also let us know default SMTP logs path to get some other details to dig further.