QB Issue Resolution:
It depends on which version/year of QB you have and which email client, but the article below should help you.
Resolution for Issue 'How do I set up email so that I can email invoices to customers?' available: Yes (Solved).
Source: Intuit Community forum.
How do I set up email so that I can email invoices to customers?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.Support for AR
Quickbooks Repair Pro will resolve it efficiently and affordably. Guaranteed resolution of your issue, or you pay nothing!
Error Recovery for AR [Canada]
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