I'm using vim+mutt to write e-mails which I send via IMAP through Google. When I include a signature file to be used, and that file contains an ASCII signature (figlet font), it is badly displayed by the (new) Yahoo web-based e-mail client (the lines look miswrapped, which makes the signature unintelligible).
However, if I copy the same signature to be included in Gmail's web-based client (in settings > use signature) and send an e-mail to Yahoo mail (in the new web-based client) using that signature, it is correctly displayed.
Another weird note is that I have set up the web-based e-mail client (that is the new Yahoo mail) to display the plain text in Courier font, but the signature still looks distorted; yet on a different Yahoo account (this time using the classic Yahoo web-based client) the figlet font signature is displayed properly, which is also the case with my web-based Gmail client if I add the Google Chrome add-on for plain text (if I don't use the add-on, the signature is, again, distorted). I'm still not sure if that has to do with the web client.