If your clients are not receiving your invoices or other email correspondence through Tally the chances are their spam filter is blocking it before it gets to them.
The spam filter stops these emails because it expects the emails to come from your actual email server however they come from Tally’s Server and thus it filters it out.
The way to get around this is to send emails in Tally on behalf of your email address but from an alternative, meaning replies go back directly to you however they are sent from a generic email address.
To set this up go to Settings > Advanced Settings and input the email address you want to Send emails from In the ‘Use on-behalf-of’. This will usually be a publicly available account (gmail) however you can use an alternate email address your clients will not have ever received an email from.
Hit ‘Update’ and you’re ready to go.