Digital signatures prove to the recipient that the content of the message was signed by you and not an imposter and that the contents have not been altered in transit, but they also increase the size of the e-mail significantly. To be able to decide when and where you want to add a digital signature go to Tools > Options > Security in Outlook and uncheck the flag "Add digital signature to outgoing messages".
Note: If you want to use digital signatures, it is best to use Microsoft Word as your e-mail editor. In this case you can choose the shortcut present on the toolbar for digitally signing and encrypting your e-mails.