550 5.2.125 The sender has exceeded the maximum number of messages they are allowed to send via client submission either per minute or per day. [HK0PR02MB2499.apcprd02.prod.outlook.com]

This seems to be a limitation in Office 365 limiting the maximumber of messages/per day and per client. 

You can refer the sending and receiving limits in the link below,

https://technet.microsoft.com/en-us/library/exchange-online-limits.aspx#RecipientLimits

https://answers.microsoft.com/en-us/msoffice/forum/msoffice_o365admin-mso_exchon-mso_o365b/office-365-message-rate-limit-smtp-client/177a5008-1b3c-423c-84d4-832c7b332172