SMTP Error (452):Failed to add recipient ... too many recipients in last hour

[#1]

Receiving mail fine but in the last day all SMTP sends via Outlook 2013 end with error 452.

I'm sending a dozen emails a day, at most, so don't understand why I would be getting this error. Have tried sending to generic gmail addresses and other business and medical organisation email domains and they all bounce.

Currently unable to work effectively, any help appreciated!

Thank you!

[#2 · Accepted solution]

OK - going to close my own issue here after scratching my head all day I found one email in my outbox with around 100 recipients..... obvious really but I hadn't spotted it. Kicking myself now.