If internal emails are being processed by exchange but external emails are not coming through most likely you have an issue with back pressure. In Event Viewer under the application log search for event id 15004-15007 to determine what issue you are facing.
- Event log entry for an increase in any resource utilization levelEvent Type: ErrorEvent Source: MSExchangeTransportEvent Category: Resource ManagerEvent ID: 15004Description: Resource pressure increased from <Previous Utilization Level> to <Current Utilization Level>.
- Event log entry for a decrease in any resource utilization levelEvent Type: InformationEvent Source: MSExchangeTransportEvent Category: Resource ManagerEvent ID: 15005Description: Resource pressure decreased from <Previous Utilization Level> to <Current Utilization Level>.
- Event log entry for critically low available disk spaceEvent Type: ErrorEvent Source: MSExchangeTransportEvent Category: Resource ManagerEvent ID: 15006Description: The Microsoft Exchange Transport service is rejecting messages because available disk space is below the configured threshold. Administrative action may be required to free disk space for the service to continue operations.
- Event log entry for critically low available memoryEvent Type: ErrorEvent Source: MSExchangeTransportEvent Category: Resource ManagerEvent ID: 15007Description: The Microsoft Exchange Transport service is rejecting message submissions because the service continues to consume more memory than the configured threshold. This may require that this service be restarted to continue normal operation.
https://docs.microsoft.com/en-us/exchange/mail-flow/back-pressure?view=exchserver-2019