Email Configuration Settings

We have an receivables application through which our subscribers can sent email reminders to their customers. Subscribers can enter the SMTP credentials like 1) SMTP Server, 2) Port #, 3) Email ID (of their company domain), and 4) Password, and the emails are sent to their customers from our application. It is working fine. But now, one subscriber has changed the email provider and it is not working. Now they are using office 365. We do not have problem with other subscribers. We need somebody who know how to fix the issue?