Moved from Windows phone of six years vintage. System no longer sup[ported so time to move on.

Have set up email on new phone - to access my Slingshot email, mainly while travelling in NZ and overseas. However, I am only able to read my emails. I am unable to send any - receiving the message "Cannot send mail. The server rejected one of the recipient's addresses. The message has been placed in your Outbox". No matter my efforts it cannot be resent (from Outbox). I have set up my access using the instructions from the ISP's website. All my parameters are correct. Have been in touch with Help at Slingshot and my settings are confirmed as correct. Have tried removing then creating account again but with no change to what is occurring. Have tried pop and imap set ups, setting ports as advised by ISP. Still no ability to send emails.

All seems odd - almost as if my ISP does not "accept" iPhone as an acceptable device. That cannot be the case. Have been back to vendor of phone (H/N) and Tech assistance there. Phone was confirmed as working OK other than the email (not) sending issue. Was told to contact ISP again as "they need to set some settings at their end". Don't think I got good advice!!

Has anyone got any ideas as to what my issue is regarding inability to send emails.

Thanks, in advance.

PS - updated our old Tab A Tablet the other day. Email set up there went perfectly and I can send and receive emails perfectly.

PPS - had no problems with Windows before I made the change. I don't believe I've made a mistake by switching O/S's - must be some minor thing I have missed. So too have Slingshot and my (so called) H/N Tech expert.