Can't send out mails and my clients are waiting on them...

Can't send out mails and my clients are waiting on them...

Last night, after using Zoho mail for almost a year on this account, I suddenly get this message: 


Unable to connect SMTP server:mail.shawhosting.ca Reason:Authentication failed:535 5.7.0 authentication failed


I have checked with my mail hosting company and it is not a problem from their end. They did think that the smtp server may be set up incorrectly perhaps. If this is the case, I have no idea why suddenly this has changed. I haven't changed a setting at all for months and months.

Can you let me know what I need to do in order to be able to send out my mails. This is really putting me behind with the communications with my clients.

Thx!

~Davida