postfix relay aunthentication error

I have setup Postfix to relay emails to our exchange 2016 server, but get an
error when i try to send:

Nov 5 15:35:00 GC-NAGIOS postfix/smtp[9093]: 3FA883E2A2D:
to=<my. ... at mymail dot>,[]:25,
delay=1100, delays=1095/0.01/5/0, dsn=4.7.3, status=deferred (SASL
authentication failed; server[] said: 535 5.7.3
Authentication unsuccessful)

The username and password are correct so i don't know why it wont
authenticate with the mail server, i have checked it is in the right format
(username:password), i don't know if there is anything i need to to do on
the exchange server side for this to work.
anyone got any ideas?