A solution you might try is, go to:

Home > Tools & Settings > Server-wide mail settings > Settings

Limitations on outgoing email messages

The following limits are default. They can be overridden in each service plan and subscription. The limits do not work server-wide, but act as a template for the plans and subscriptions for which no custom limits were specified. Note that switching on the limitation of outgoing email messages can affect the mail server performance. Learn more about limiting outgoing mail

Disable this: Turn on limitations on outgoing email messages