An SMTP credit is calculated as one message sent to a single recipient. If the number of recipients on a single message is greater than one, each additional address uses a SMTP credit. 

• On our entry level SMTP service this means you can send 150 individual messages to 150 different recipients. 1 * 150 = 150
• You could also send 15 messages, each with 10 recipients - 15*10 = 150 credits. 
• Each recipient either in the To:, cc: or bcc field is counted as a credit.