Sometimes, the default maximum message size (including attachments) in Exchange is not enough (30 MB is the standard). To increase this to 100 MB, do the following:

First check the current maximum size on all relevant Exchange components:

Get-TransportConfig | ft MaxSendSize, MaxReceiveSize
Get-ReceiveConnector | ft Name, MaxmessageSize
Get-SendConnector | ft Name, MaxMessageSize
Get-Mailbox Administrator |ft Name, MaxsendSize, MaxReceiveSize

Then set it to 100 MB for all components:

Set-TransportConfig -MaxSendSize 100MB -MaxReceiveSize 100MB
Get-ReceiveConnector | Set-ReceiveConnector -MaxMessageSize 100MB
Get-SendConnector | Set-SendConnector -MaxMessageSize 100MB
Get-Mailbox | Set-Mailbox -MaxSendSize 100MB -MaxReceiveSize 100MB

That's it. Be aware that attachments are encoded before they are sent. This encoding adds about 30% of the size. So in reality, with the 100MB setting you can send and receive attachments that are 60 to 70 MB in size.