This could be due to IP blocking / blacklists. This is a huge problem these days, and many emails are simply dumped by stupid systems that use spamcop etc IP blacklists.
An email sent using a webmail facility originates from the ISP owning the facility. If you live in the UK and are travelling, the email will appear to be coming from that UK ISP (whose website you are logging into for the webmail).
However an email sent via a GPRS/3G connection will originate from the GSM network provider (I am assuming that you have configured their SMTP server in your email software). Some of these providers are routinely blacklisted, especially some in the 3rd world where GSM data is so cheap (15p per MB for example in South Africa, Vodacom) that spammers can use it.
However if you are logging into an authenticating SMTP server (corporate users tend to use them) then you can be on GSM and the above para doesn't apply...