That is not the error message you see. Do not describe the error
message, do not summarize it, do not paraphrase it, do not truncate it.
Show the ENTIRE error message.
My guess based on the dearth of information (like not mentioning the
TYPE of e-mail account or even the Outlook version) is that you did not
configure the e-mail account defined in Outlook to authenticate to the
SMTP mail host.
Usually you get one chance per potential respondent to elicit a reply
from them. If they skip your post because you gave them nothing to go
on (no details, no versions, no context), usually they will just move on
to the next post and never return to yours. Go read: