Because Outlook cant query a SMTP for the maximum attachment size, it uses a default attachment limit of 20 MB (20480kb), which is about right for most ISPs. This count includes local folders that aren't part of the online account.

'Exchange Online customers who need to send legitimate bulk commercial email (for example, customer newsletters) should use third-party providers that specialize in these services. In order to reduce NDRs and failed sends, Outlook checks the message size before sending and alerts the user that the message is larger than the mail server allows. 10,000 per profile across mail, calendar, contacts, tasks, and notes. They have stated that Office 365 is not meant to be a bulk email service. NOTE: Microsoft has implemented Receiving and Sending Limits. Storage - User Mailbox, Shared Mailbox, Resource Mailbox

Note: when you attach a file to a message, the file size of the attachment increases (it can increase as much as 30% in size when it is sent). Message send/receive size limit (including attachments) - Keep in mind that someone's mailbox on the Office 365 server will be prohibited from receiving messages if they exceed their maximum quota limit. While the University of Nebraska email systems allow sending and receiving 150mb email messages (including attachments), other external email systems continue to have smaller limits.Email attachments count against overall email storage quota.Message size limits in the users mailbox werent set, setting them to 100 MB didnt make a difference. SysTools Outlook Mac Exporter extracts all emails from Outlook 2011 Mac file and convert into PST file format, Outlook 2019, 2016, 2011 to PST Converter is a perfect tool for Mac file conversion. This can increase the time required to send the message. Message size limits on the Exchange server transports are 100 MB.

Attaching a large file to an email message requires uploading that file to the email system.This includes both the message content and attachments. Maximum total message send/receive size is 150mb.Below are some of the limits for Office 365 accounts - more detailed information can be found at Exchange Online Limits

Email file size limit outlook for mac