Hello @littlegemresorts ,
This would be a trigger caused by the contacts' device / email settings. If they're using a PC, then they will most likely cause whatever internal mail / Outlook program is already on their device, unless they've changed its settings. This is common practice across pretty much any device capable of opening mailto links. Apple devices will try to default to Apple mail unless their settings have been altered, PC's will typically default to Outlook or a similar Microsoft email program, Androids to their default email app, etc.
I'd suggest you recommend the affected contacts update their device and/or email settings to trigger mailto links to only open for their preferred email client. Or you can recommend that they just copy-paste the mailto email address and open their preferred email client / app on their own,
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