SMTP error sending 1st invoices only, all others okay

[chrisp]

After two years of using WHMCS client invoices and system emails are no longer being sent out. This coincided with an upgrade to version 4.2.1. but I'm not sure it's related or not.

 

Each time the cron runs I get an SMTP error like this:

 

---------------------------------------------------------

"Cron Job: Daily Email Backup - Sending Failed - SMTP Error: The following recipients failed: xxx@xxxx.com

SMTP server error:"

 

or ...

 

"Admin Email Notification Sending Failed - SMTP Error: Data not accepted.

SMTP server error:

(Subject: WHMCS Cron Job Activity)"

---------------------------------------------------------

 

All first and subsequent Overdue Notices go out fine, as well as other invoices and emails that we manually send from WHMCS, but not these first automated invoices.

 

WHMCS support just says to check the mail server, which I've done repeatedly.

 

Has anyone had an experience with this before?

 

Thanks,

Chris