I am getting the following message in the log when attempting to send emails outside Appian users like a gmail account.
Different recipient domain on a smart-hosted and non-authenticated email. Sending server should retry this recipient separately next.
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A lot of times, Appian finds it hard to convert normal text to an email address. Can you try wrapping your email address in toemailaddress() function and see if it works?
I think this is more a mail server setup issue. I tried to search the internet for that message and found some indications in that direction.