So via the email users tab how does it send? Is it by your own servers? As my vBulletin is set up with sendgrid but they don't seem to be sending via those SMPT's!
Depends how aMember is set up in configuration. Cannot see how aMember will affect how you have set up the way it sends emails.
aMember by default send e-mails via your webhosting's default mail server. You can change it at aMember CP -> Setup -> E-Mail