If QuickBooks displays the QBMapi64 Out of Memory Error while sending emails, check your system’s available memory and update QuickBooks and Outlook. Corrupt MAPI32.dll files can also cause this issue, so repairing or reinstalling them may help. Running QuickBooks as an administrator and clearing system cache can resolve the problem quickly.

https://www.apsense.com/article/841213-address-the-qbmapi64-out-of-memory-issue-with-useful-fixes.html