Communication limits are a feature in Marketo that prevent you from over-emailing your leads by setting a maximum number of emails they can receive per day or per week. Communication limits automatically apply to all email and engagement programs, which are two types of programs that send emails to your leads. Email programs are used to send one-time or recurring emails, while engagement programs are used to send a series of emails over time based on a predefined schedule and logic. You can enable or disable communication limits for each email or engagement program by checking or unchecking the box that says “If a person has reached communication limits, block non-operational emails”. Operational emails are emails that are marked ascritical or transactional and should always be sent regardless of communication limits, unsubscribes, or marketing suspensions. Communication limits do not apply to other types of programs, such as default, event, or webinar programs, unless you use smart campaigns within those programs to send emails. Smart campaigns are a way to define the logic and actions for your marketing activities, such as sending emails, updating lead data, adding leads to lists, etc. You can enable or disable communication limits for each smart campaign by checking or unchecking the same box as above. You can also set the default behavior for smart campaigns in the Admin area, under Communication Limits. References:

Enable Communication Limits

More Than You Ever Want To Know About Communication Limits

Email Programs

Engagement Programs

Operational Emails

Smart Campaigns