Error: authentication failed

[rogerrr222]

Hi there,

I've been using listmonk for a while and only lately something doesn't work:

I get an error message when I send...

• opt-in emails for subscribers
• and campaign test messages

This is the most recent error taken from the log:
2025/09/15 18:37:57.257445 manager.go:501: error sending message in campaign Newsletter Sep 2025: subscriber 5: 535 5.7.8 Error: authentication failed: UGFzc3dvcmQ6

At first I received a heads-up by somebody who wanted to subscribe via a link. When I reproduced it, it prdouced a "processing error". Afterwards I tried the things mentioned above.

I don't have any clue what it means and what to do. Anyone familliar with this?

Kind regards