In this instance, its best to build the desired email signature directly into the email template, rather than using the personalisation token - then you can control the signature the recipient sees fully.
Override contact owner signature for marketing emails
SOLVE
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Q: [How to] Override contact owner signature for marketing emails?
Short A: Don't use personalization tokens.
Longer A:
Sounds like you need to remove personalization tokens and simply 'hard code' a signature at the end of your newsletter for your use case.
Personalization tokens are used to vary content according to each contact record -- while still allowing for a single default value if the personalization token property used is blank. Personalization tokens don't support multiple default values.
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In this instance, its best to build the desired email signature directly into the email template, rather than using the personalisation token - then you can control the signature the recipient sees fully.