For two / three days, some customers have complained about not receiving an answer to their emails.

Checking our mailbox we do not see that any of them have been bounced and looking in the control panel (problem solving) we observed that after several attempts the zoho records indicate that they have been delivered.

We note that the message (more or less similar in all) is:

1. 4.7.500 Server busy. Please try again later from [136.143.188.53]. (AS77713295) [AM5EUR02FT055.eop-EUR02.prod.protection.outlook.com]

And from what we have googled, it may be a communication problem with the Outlook / Office365 servers.

We use Thunderbird as a desktop mail manager.

Can any of you guide us on how to solve this problem? We're small company and we've not experience dealing with email servers.

It is causing us additional work by having to confirm the receipt of important emails.

Thanking your help.