Is there a web standard there email service providers follow or is it only based on experiments?
Currently yahoo has truncated our subject line and I could not find an accurate article which says the character limits on the subject line?
We tested it on different mobile phones but all was ok except for the subject line on their desktop. Does anyone know a reliable source where we can find such article or guidelines?
I'm afraid there isn't a set of standards that are used by all email service providers, it would certainly make email marketers life easier if there was! We're supporters of a project that is working towards a set of standards which you can read about here. At this stage there is lots of work to do to ensure your email looks good across all email programs, sorry about the hassle here.
Although I don't know of a specific character limit on subject lines with Yahoo, there have been some cases where subject lines are truncated. We have found that Yahoo imposes a limit of 18 characters when certain international character sets are used, is this a similar issue to the one you experienced?
At the moment, we don't have a clear idea of why this occurs I'm afraid. If you are able to contact support, we may be able to find solutions for your specific case.