outlook 2016

when I am sending calender invite from my outlook  2016

I am getting undeliverable notification groups:
The format of the e-mail address isn't correct. A correct address looks like this: someone@example.com. Please check the recipient's e-mail address and try to resend the message.

like this I am getting 8 or 9 undeliverable notification for different email messages.

but when I send next time it goes through with just the email address

any reason why =SMTP: is coming beforeemail address
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Sunil ChauhanLead AdministratorCommented:
this address probably in your outlook address list cache, probably try deleting and try again.

to delete, open new msg, start typing the address, as soon as it appears delete it and then try sending.
pramod1Author Commented:
but my question is why I got =SMTP:wrahoich@gmail.com' and then next time I sent to same address without =SMTP:  it went fine
pramod1Author Commented:
Why this popped up with email address;=SMTP:
Sunil ChauhanLead AdministratorCommented:
as you are using the outlook, I suspect that might be the cached address is ambiguous.

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pramod1Author Commented:
I didn't delete cache address next time I sent email , it went fine
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