Sending E-Mail to a client with an Underscore ("_") in the address fails

Sending E-Mail to a client with an Underscore ("_") in the address fails

I have two clients that have e-mail addresses that contain underscores in the e-mail address ( First_Last@client.com). I can not send replies or ask questions to them in the service desk without the send failing. Cline's without an underscore in the e-mail address work just fine.

I've tested this independent by setting up two free mail accounts through MSN and the Underscore address fails while the non-underscore address succeeds. It does not seem practical to ask a client to change their mail address format - are there any work around or ways to solve this problem in SDP - MSP?

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