Outlook shows "Please wait to send" when sending from New Outlook or Outlook on the Web.
When attempting to send an email using Outlook New or Outlook on the Web with an inserted signature, you may see the following message after a Signature 365 signature has been inserted:

This can occur when the signature contains embedded images. The behaviour is controlled by Outlook and Exchange Online, not by Signature 365.
Reason
This issue can occur due to a Microsoft change to prevent broken images from being displayed in the Web versions of Outlook when embedded images are used. Previously, a broken image could be displayed due to Outlook's underlying image handling process.
Microsoft updated the image insert process to ensure the image is available in Outlook, but this also requires the image to be stored within Exchange Online after it is inserted. This results in a potential delay whilst the image is confirmed saved by Exchange, and is caused by the mailbox being accessed online.
As such, Outlook prevents skipping the message due to the potential for an email to be sent when the embedded image is not stored in Exchange.
Workaround
It is possible to enable hotlinking for a customer tenant, which causes images to be referenced as opposed to embedded in the email itself.
We detail the potential benefits and drawbacks of embedding and hotlinking in the article below:
Using hotlinked vs embedded images
If you would like to enable hotlinked images for your tenant, please contact our support team at support@symprex.com
Outlook (Classic) agent release notes
See release history, fixes, and updates for the Signature 365 Outlook Classic agent.
Using a group policy object to disable signatures in Microsoft Outlook
Use Group Policy to disable Outlook signatures and prevent users creating their own signatures alongside Signature 365.