unable to send outbound email when on Three network

[darrenings]

Hi,

I've been have trouble for a number of years now with sending email over my boat Wi-Fi (which is on the UK Three mobile network). Typically from my iPhone I send an email and i get the error "Cannot send mail - "email address" is not valid and has been rejected by the server". I think this is because the IP addresses I am using are in a blacklist because of SPAM. The email then sits in my outbox, and what I usually then have to do is come off Wi-Fi to Vodafone and then send it again.

But I'm a little confused by this, since I am using authenticated SMTP to send these emails, but aren't they trusted? Is there a setting I need to play with to fix this?

Thanks
Darren