I have a tpg.com.au email and it seems to be blocking messages from hotmail.com. Below is extract of bounce back message received from hotmail.com:
Diagnostic information for administrators:
Generating server: HK2PR01MB3396.apcprd01.prod.exchangelabs.com
Receiving server: HK2PR01MB3396.apcprd01.prod.exchangelabs.com
Total retry attempts: 14
5/1/2021 10:14:54 PM - Server at HK2PR01MB3396.apcprd01.prod.exchangelabs.com returned '550 5.4.317 Message expired, cannot connect to remote server(421 Service not available, closing transmission channel)'
5/1/2021 10:07:28 PM - Server at tpg.com.au (22.214.171.124) returned '450 4.4.317 Cannot connect to remote server [Message=421 Service not available, closing transmission channel] [LastAttemptedServerName=tpg.com.au] [LastAttemptedIP=126.96.36.199:25] [SG2APC01FT008.eop-APC01.prod.protection.outlook.com](421 Service not available, closing transmission channel)'