Such an error may arise due to the following:
Incorrect email setting
Damaged MAPI32.dll file
QuickBooks running as administrator
Damaged QuickBooks installation
Damaged Outlook installation
Outlook open in the background while sending email via QuickBooks
Please note that the first solution itself may help you resolve your problem so do not follow the other two solutions in case it happens so.

Solution 1: Make sure that QuickBooks is not running as an administrator
Right-click on the QuickBooks icon, then choose Properties.
Press the Compatibility Tab
Deselect the Run this program as Administrator
IMPORTANT: If Run this program as Administrator is grayed out, then you must first choose the “Show Settings for All Users“.

Tap on Ok.
Exit, then restart QuickBooks.
Solution 2: Make sure that your email preference is set properly in QuickBooks
IMPORTANT: In case you are using the Microsoft Outlook with Microsoft Exchange Server, open Outlook and sign in before checking the QuickBooks preference.

Click on Edit > Preferences >Send Forms.
Choose My Preferences tab, set Send e-mail using to the option that you want to make use of, then click on OK.
If the preference is set correctly, to toggle it:
Click on Edit > Preferences >Send Forms.
Press My Preferences tab, select QuickBooks E-mail, then tap on OK.
Click on Edit > Preferences.
Select Outlook, then, tap on OK.
Exit QuickBooks and all other programs.
Restart Windows& then QuickBooks
Email the report or transaction.
Solution 3: Make sure that your email preference is set properly in Internet Explorer
Exit QuickBooks.
Open Internet Explorer.
Click on Tools >Internet Options.
Choose the Programs 
In the Programs tab, ensure that the correct email program is chosen as the default email program. For instance, when you are preparing to send an email via Outlook, then ensure that only Outlook is chosen. However, if you do the same through Outlook Express, then be sure that only Outlook Express is selected. Select the correct option, then choose Applyand tap on OK.
Exit Internet Explorer.
Open QuickBooks.
Email the report or transaction.
Solution 4: Test and repair your MAPI32.dll
Important: These steps maybe best performed by an expert or a professional as these are highly technical.

Use Word to test MAPI32.dll functionality
Restart your device to exit and reopen Windows.
Open a blank document in Microsoft Word.
Click on File > Send >Email as PDF Attachment.
If the email opens with no errors at all, contact our QB experts for assistance. You may contact QuickBooks Pro Support
If the error arises while attaching the document, contact Microsoft Help and Support to fix possible problems with Outlook or Office Suite and the MAPI settings.
When you have resolve your problem and emailed from Microsoft Word, then email the report or transaction from QuickBooks.
Repair MAPI32.dll
Exit all open programs.
Choose Windows Start, thentap on Computer.
Visit the C:\Windows\System32.
Double-click on exeand follow any onscreen instructions.
After exe finishes running, restart your device.
Rename MAPI32.dll
Perform this if repairing MAPI32.dll does not fix the problem.

Exit all open programs.
Using File Explorer, visit the C:\Program Files (x86)\Common Files\System\MSMAPI\1033.
Right-click on MAPI32.dll, then rename it to dll.OLD.
Restart your device.
Access exe again.
After Fixmapi.exe completes running, restart your computer.