Many providers like GoDaddy who host email accounts, limit the smtp quota - the amount of emails you can send each day. It is usually a reasonable amount like 250. That can be a problem if you are using eM Client’s mass mailing feature. However, you can probably purchase options to increase the limit.
But if you add all the recipients as receivers of a single email, that solves the problem, though some email providers also limit the number of recipients on a single email, like 100 for GMail.