Is volume a problem?

Volume in an of itself is not a problem. Companies sending mail people want can send multiple emails a day to every user. The volume isn’t a problem because the mail is wanted.
Many senders are confused and think volume is a filtering criteria. It’s not. Send all you want; just send it to people who actually want the mail.
A lot of companies in their growth phase find they do have delivery problems as their volume ramps up. But the problem isn’t the volume, the problem is that mail programs don’t scale. Companies mailing lower volumes can get away with sloppier practices. One because the chances of hitting bad addresses increases with the number of addresses you have. But the other is that filters do take volume into account. It’s not that the volume directly causes the filters to trigger, but volume causes the filters to look harder at mail. If the reputation and metrics are good, the mail is fine and hits the inbox. If they are poor, then mail hits the bulk folder or is filtered.
Overall, volume isn’t a problem, but increasing volume can expose fundamental problems in a mail program that result in delivery issues.