The salesforce plugin has a limitation of a relatively small message size limit of 32kb,
The proposed solution in this article already seems to be already the default in Outlook, but good to check:
If you are copying and pasting from a template in Word, especially if it is heavily formatted with tables and links and various formatting, the message size can quickly become to large for the plugin to upload to salesforce. Also if it is combined with a signature that has heavy formatting and includes embedded images, this will add to the size.
A workaround for copying from Word documents is to always do Merge Formatting when copying and pasting, (right click paste-merge formatting).
Also, create a copy of the signature without any embedded images and use that for Salesforce related emails. This should hopefully reduce the message size enough that it can be captured into salesforce in full.