This issue has been resolved by the Exclaimer team.
The underlying cause was actually due to an email delivery delay on the Microsoft Office 365 platform.
All signature stamping is now operational as expected.

An issue with Exclaimer Email Signatures has been identified on the Australian server. This has been brought to our attention by Exclaimer Cloud.

Imprinting of signatures may not occur for all emails, however the incident report received from Exclaimer indicates that stability is returning.

Full details can be viewed here: http://stspg.io/56104d3e3