Earlier this year, a report surfaced that suggested Yahoo! Mail was the cause of spurious data usage by Windows Phone and iPhone owners. The root cause was the Yahoo! IMAP mail servers, which were transmitting more data than was necessary for each email request. Occasional email checking was not problematic, but individuals who checked their email frequently could easily accrue enough data to push them over their plan's limit. Yahoo! confirmed this problem, told affected users to check email manually and promised to work on a solution.
A month after this discovery, Yahoo! has reportedly pushed out an update that fixes this issue. Rafael Rivera, who initially identified the culprit as Yahoo! Mail, now reports that Yahoo! sends back the appropriate information during an email fetch request. Instead of sending the full message header, the Yahoo! mail servers now send back only the message ID. All you iPhone users with a Yahoo! mail account and a limited data plan can now check your email freely.