Customer doesn’t know his email

So I work at a hospital in the US and got a call from a gentleman last night

$Me = me

$Gent = the gentleman

this will be somewhat paraphrased

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$Me “my opening riff”

$Gent “Hey I am trying to log into my email and it is giving me an error logging in”

$Me “Ok, did you have trouble logging into the computer?”(this is because those passwords are linked)

$Gent “No I logged into the computer just fine but its not allowing me into the email”
$Me “Alright, can i have you tell me what you are entering for the email”

$Gent “first.lastname@companyname.”

$Me “alright I see whats wrong here you need your email to read first.lastname@companyname.com“

$Gent ” alright ill try that”

$Gent “It still doesnt work”

$Me “did you add the c-o-m after the last period?”

$Gent “No? Why would I do that?”

$Me “Can you please c-o-m to the end of your email?”

$Gent “Fine but its never worked that way before”

$Gent “Alright I was able to get in… Have a nice day”