I built similar VBA about a month ago and didn't have this type of issue. I don't understand what you mean about an errorbox in Outlook. Could you maybe add a picture?

Im sorry it is in Danish, but it says something Like:
"A program is trying to send a mail on your behalf"
"If this is unexpected, click on "Afvis" (Reject) and check that you antivirus program is updated."

"Click on help to get more information about mail security and about how you miaby can avoid having this warning displayed."

I have checked if the antivirusprogram is uodated, which it is to the best of my knowledge - if i click on "Tillad" (Allow), the mail is send.

Send Email shows how to send email using VBA code.
Since this involves adding a reference to the Outlook Object Library (early binding), also checkout the video Late Binding

The Access Email Seminar shows pretty much everything you need to know about sending email from Access.