Sending mail through different provider

Hi,

When using Pegasus Mail at home using my laptop, I send and receive through my usual ISP, which is Vodafone. When I am somewhere else and connect through a different ISP, I am able to receive but unable to send messages. For example, I am in a rental home with no broadband right now and use my mobile phone as a portable Wi-Fi hotspot. I receive messages alright, but when I try to send anything I get the following error message:

Connection established to 203.97.33.229
>> 0035 220 smtpin1.paradise.net.nz ESMTP
<< 0021 EHLO

[192.168.43.3]
>> 0029 250-smtpin1.paradise.net.nz
>> 0014 250-8BITMIME
>> 0019 250-SIZE 31457280
>> 0022 250-AUTH PLAIN LOGIN
>> 0022 250 AUTH=PLAIN LOGIN
<< 0052 MAIL FROM:<rouke.bakker@paradise.net.nz> SIZE=2028
>> 0029 530 Authentication required

How can I ensure that I can send email messages from my laptop using Pegasus Mail regardless of the ISP I connect through?

Regards,

Rouke