Some of them may not reach you. This is to protect against „fake“ emails that are used for spamming.

What do I call a „fake“ email? It's an email that I send from an account on my domain (blueboard.cz), but I set the sender as, for example, zeman@hrad.cz. This is very easy to do, any email client can do it. Yes, you too can have an invitation from Zeman for a drink in your inbox tomorrow.

And how does this relate to forwarding an email? Forwarding works because the forwarded mailbox receives the email and forwards it again. In doing so, it leaves the original sender and does not acknowledge itself as the sender. So the email is actually „forged“.

One day, providers decided to defend against fake emails. So they started to check if the server the email comes from matches the sender's domain (explained at the end of the article). If not, they don't deliver the emails to their clients' inboxes and throw them away. Including, of course, any forwarded ones.

If I explained the previous paragraph clearly, it should be clear that if the forwarding mailbox and the destination mailbox are with the same provider, the email will always be delivered. If you have that, you have nothing to worry about.

How to solve this then?

Ideally, do not forward emails. Put yourself in the sender's shoes. Send a message to email A and someone will reply from email B.

Cancel your forwarding and connect to other inboxes directly from your email client. Most clients can already consolidate accounts into one inbox, so the convenience should remain.

How does this work from a technical point of view?

How does the recipient server verify that the email is coming from the correct server? They use something called SPF records to do this, which are written to the sender's DNS domain. In such a record you write „The sender's mail server must have IP address 1.2.3.4“ (of course this is written in special syntax).

So the recipient's server sees the IP the email is coming from and asks the SPF record for the allowed IP. If it doesn't agree, it rejects the email or marks it as spam. And for a forwarded email, those IPs don't match because the original sender's mailserver has a different IP than the mailserver that forwarded the email.