We have seen some customers having trouble sending email from their mobile devices, but only (or mostly) when connected via the carrier (that’s the phone network, ie: Vodafone, Orange, …) and not when using wifi.
The problem can sometimes be that the IP address you’re being assigned by your ISP (the carrier in this case) is blacklisted by common spam services. Every time you connect to the internet via the mobile network you get a so called “dynamic” IP address, and this address will have been used by many of their customers. This IP addresses can have a “bad reputation” and get temporarily blacklisted.
Unfortunately it is the ISPs responsibility to maintain their IPs reputation, and they are responsible for requesting the addresses be delisted.
To work around this problem you can try reconnecting (you may get a new IP address that’s not blacklisted) or you could use our webmail interface to send email while on the go. The webmail interface does not suffer from this problem as emails are sent directly from our servers and not from your carrier assigned address.
Just in case you may also want to check that your mail client is configured correctly.