Unable to send mail thru SMTP after a while (IMAP Mail on Mac)

Unable to send mail thru SMTP after a while (IMAP Mail on Mac)

Hi!
I have an issue for a new client using your ZoHo mail. I have done all the steps, configure domain, enable IMAP etc etc.

It seems the Apple Mail client work for some time, then when trying to send e-mails the smtp.zoho.eu server returns a 5.5.1 auth error. The account has all the correct information, no 2-way auth etc.

It works fine to receive e-mails but outgoing e-mails are suddenly not working any more?