From the rules on movement modes:
A creature with a fly speed can move through the air at the indicated speed if carrying no more than a light load. (Note that medium armor does not necessarily constitute a medium load.)
Unlike walking, there's no rule that lets you move slower with higher loads; if you exceed light load, you just can't fly. If you need a more explicit prohibition rather than mere lack of permission, consider the feat Reinforced Wings from Races of the Dragon, which says:
Normal: Flying creatures can't fly in medium or heavy armor or when carrying a medium or heavy load.
Now, a raven has Strength 1 and is Tiny. Being a familiar doesn't change either of those properties. So look that up on the carrying capacity chart, which says that Str 1 has a light load of "3 lb. or less", and Tiny multiplies that by ½.
Conclusion: your raven can carry at most 1.5 pounds while flying.
Even if carrying capacity weren't a problem, the distance from Thay to Waterdeep is about 2400 miles. A raven's 40 ft. base speed translates to overland travel of 32 miles per day. So a round trip would take 5 months.