Cannot send emails when away from home

[#1]

When ever I am out and about, (away from my home wi-fi) I cannot send any emails from my iPhone, whether I'm logged into a third party's internet or on 3G, 4G or 5G.  Once I get home and on my home wi-fi, the emails will then be sent.

The message states a Server error. 

I have checked the email settings on my iphone and they match what Plusnet advise.

Any advice please?

[#2]

Do you use a computer at home, or are you on a phone/tablet only? If you use a computer, does it have nan email program on it, or do you just rely on webmail?

[#3]

@Fotoman you need authentication enabled on the outgoing server setting.

[#4]

Everything works fine on my PC. It's just the iphone.

Authentication; 'Password' is checked.

Use SSL is ON

[#5]

Everything works fine on my PC

but presumably that is on your home network ?

When not on a Plusnet network e.g a 4g mobile , you must use. authentication on the outgoing SMTP  server (relay.plus.net)

Even then you may get transient problems with IP reputation if the network uses CGNAT ( most mobile networks do )