I make heavy use of Claws Mail's useful address tab-completion in to/cc text boxes, and fear that I'll erroneously send to an unintended address.
Erroneously sending to addressees within my company wouldn't be too problematic, but sending externally i.e. to a public mailing list / customer, etc. could be quite embarrassing, despite using GPG for truly private mail.

For this reason I'd love to have the ability to have the to/cc addressee fields colour-highlighted, based on whether the addressee's domain matches a user-defined filter.