A forwarded message arrives at your provider from our servers, not from the sender's. Everything else about it, the From header included, still says the original sender. That mismatch is exactly the shape of a forged message, and filters are right to be careful about it.

We do everything a forwarder can to make it fine: our servers are authenticated for the domains they send from, forwarded messages are rewritten so the authentication checks pass, and our sending reputation is monitored daily. The remaining decision belongs to your provider.

The three fixes, best first

• Mark one as not spam. In Gmail, Outlook or anything else, this teaches your provider more in one gesture than any setting we could change.
• Add the address to your contacts. Mail from a known contact is treated differently by every large provider.
• Add a rule at your provider that never sends mail from our servers to spam. In Gmail this is a filter with "never send it to spam" ticked.

If it is only one sender

Then the forwarding is probably not the problem. Some senders publish a strict DMARC policy and sign nothing, in which case their mail is fragile the moment it passes through anybody. Reply to them from the alias once: a message you sent is the strongest signal your provider has that the correspondence is wanted.

If it started suddenly

Check whether the messages are large or carry attachments, whether you have just crossed into a new provider policy, and whether the sender changed their sending platform. If several of your addresses started at once and nothing changed at your end, tell us: a shared domain problem is something we can see and act on, and we would rather hear it early.