Is it possilbe to modify outgoing email message from Apex? I mean capture all outgoing emails (coming from SF standard features, like Mass emails or single email sent via Activity) and modify them 'one the fly' before sending (just like in triggers for other objects). There is EmailMessage object, but that appears to be only used for incoming emails. I would like to modify users signature in an email (remove html tags when body is a plain text).
Short Answer is, No. These is no single rout through which all email sent and we can catch and modify by accessing that route.
However, you can acheive it partially via Apex code. Build a class over
EmailMessaging and send all message through your custom class so signatures can be added. Otherwise Apex doesn't provide code access to email going.