This page describes what to do in case of an Icinga alert. For more information you could search the govuk-puppet repo for the source of the alert
AWS SES quota usage higher than expected
Likely suspect for elevated email volumes is Errbit emailing too much errors.
Other things that use the same SES account include Whitehall, Publisher, Signon, Licensing and machine email.
Hitting the quota may cause emails to not be delivered.