Currently, you can only choose inbox providers that will be included in your seed list. If the seed list appears larger than you expected, in the manual test, you can choose the option to display a split seed list – several files with 25 emails on each. Send the same message with the same id string to each list. The results will be added to the same report.
For smart tests and automatic tests, you can apply email throttling — set a delay between the messages sent to the seed addresses.