Due to higher security standards we have activated GeoIP security on our servers that allows to send e-mails from predefined countries, or more precisely IP addresses. By default it is allowed to send from the countries of V4 (CZ, SK, HU, AT), and in case of foreign account, its home country (e.g. Netherlands for NL customer). Before you travel abroad we would recommend to check if your destination country is whitelisted in the GeoIP, otherwise it will be impossible to send e-mails.
Whitelist is always free to edit in the WebAdmin as shown this guide.