Notifications not working

I'm using Traccar 6.1 My notifications are not working. I tried sending an announcement from the web interface but see this error message:

535 Authentication credentials invalid - AuthenticationFailedException (... < SmtpMailManager:154 < *:99 < NotificatorMail:44 < NotificationResource:129 < ... < OverrideFilter:49 < ...)

I have checked the smtp credentials carefully but they are correct. I can't see any error in the log file.
These are the relavent entries in my xml file :

    <entry key='mail.smtp.host'>smtp.1und1.de</entry>
    <entry key='mail.smtp.port'>587</entry>
    <entry key='mail.smtp.starttls.enable'>true</entry>
    <entry key='mail.smtp.from'>m.eggleston@goldspur.co.uk</entry>
    <entry key='mail.smtp.auth'>true</entry>
    <entry key='mail.smtp.username'>m.eggleston@goldspur.co.uk</entry>
    <entry key='mail.smtp.password'>xxxxxxxxxxxxxxxx</entry>
    <entry key='notificator.types'>web,mail,sms</entry>
    <entry key='notificator.sms.manager.class'>org.traccar.sms.HttpSmsClient</entry>

Can anyone throw light on my problem please ?

The error message seems pretty clear.

I agree but I can't understand why it's happening. The credentials in the xml file haven't changed and were working previously. The same credentials are working in my email client - Thunderbird. I don't know what's changed.

Not sure what changed, but the error is coming from your email service directly.

Okay thanks Anton. I'll probe further.