NPM can monitor the network bandwidth utilization of your devices, interfaces, and applications using SNMP and NetFlow protocols. Network bandwidth utilization is the amount of data transmitted or received over a network link in a given time period. High network bandwidth utilization can indicate network congestion, packet loss, latency, or other performance issues that can affect the email delivery speed. By looking at the network bandwidth utilization of the devices and interfaces involved in the email communication, you can identify the possible bottlenecks or sources of network problems that might be causing the email slowness. References: Monitor network bandwidth utilization in NPM, Network Bandwidth Analyzer Pack, How to Monitor Network Bandwidth Using the Command Line.

Account limitations are a way of restricting the access of Orion user accounts to specific network objects, such as nodes, interfaces, groups, maps, or alerts. You can use account limitations to prevent a user from seeing or working with select monitored entities through the SolarWinds server. For example, you can limit a user to see only the nodesin a certain group, or only the maps that they have created. You can apply account limitations to individual users or user groups, and you can choose from predefined limitations or create custom limitations based on custom properties. The other options are not effective ways of limiting user access to monitored entities, as they either affect the monitoring status of the entities (unmanage), the permissions of the entities themselves (configure permissions), or the alerting scope of the entities (configure alert limitations).
References:
Limit user access to network areas with account limitations
Predefined Orion Platform account limitations
Create custom account limitations

NPM provides a default report called Top 25 Interfaces by Percent Utilization that shows the interfaces with the highest average utilization over the selected time period1. This report can be easily edited to change the number of interfaces displayed, the time period, the sorting order, the layout, and the filters2. To create a report that shows the top 50 interfaces by percent utilization, you can simply edit this report and change the Limit option from 25 to 503. This is the most time-efficient way to create the report, as it does not require creating a new report from scratch, writing a SQL query, or running the report multiple times for different timeframes. References: 1: Network Performance Monitor Administrator Guide - Top 25 Interfaces by Percent Utilization 2: Network Performance Monitor Administrator Guide - Edit a report 3: Network Performance Monitor Getting Started Guide - Customize a report

An alert limitation is a filter that restricts the scope of an alert to a specific set of objects, such as nodes, interfaces, applications, or groups. You can use an alert limitation to create an alert that only applies to a select set of nodes in your network, and exclude the rest. To set an alert limitation for the nodes to be included in the alert, follow these steps1:
Click Alerts & Activity > Alerts > Manage Alerts.
Find the alert that you want to edit or create a new alert.
On the Trigger Condition tab, define the condition that triggers the alert, such as Percent Loss is greater than or equal to 50.
On the Scope of Alert tab, select Limit the scope of this alert to a specific subset of objects.
Choose the method to define the subset of objects, such as Select objects manually, Use a custom SWQL query, or Use a custom SQL query.
Select or enter the criteria to filter the objects, such as the node name, IP address, group, custom property, or any other attribute.
Click Add Selected Objects to add the objects to the alert limitation.
Click Next and complete the rest of the alert configuration.
You can now save and enable the alert, and it will only notify you when packet loss meets or exceeds 50% for the nodes that you have selected in the alert limitation. References: 1: Limit the objects an alert applies to