Hello,
To do this, you can enter any domain name in the POP3 server field and save the settings. For example:

Then edit file /etc/config/email_to_sms via the CLI:
root@Teltonika:~# vi /etc/config/email_to_sms
config pop3 'pop3'
option limit '5'
option enabled '1'
option ssl '0'
option host '192.168.3.4'
option port '80'
option username 'user'
option password 'password'
option time 'min'
option min '1'
option modem_id '3-1'
To apply changes, restart "email_to_sms" service:
root@Teltonika:~# /etc/init.d/email_to_sms reload
Regards.