This issue can occur if the user has "Always keep a copy" selected for the "Save mail that you sent" option under File > Preferences > User Preferences > Mail. If changing this setting to not keep a copy allows the user to successfully send the message, this would indicate a problem with the Sent folder being corrupt. Running fixup, compact and updall on the mail database should resolve the problem in this case.
Also read info here: https://www.repairtoolbox.com/lotusnotesrepair.html
Regards