If you are having problems having VPOP3 forward incoming messages to an external address, but sending messages directly to that address works OK, and forwarding internal messages to that address works OK, then the problem may be the SMTP return address which VPOP3 is using.
By default, VPOP3 will use the original return address of the message, so if a message is sent to you from firstname.lastname@example.org, VPOP3 will forward the message out, using that address as the return address. Your ISP may be rejecting or discarding the message because it doesn't think that email@example.com should be sending mail through your account.
In this case, go to Mail Connectors → Mail Senders → (your Out Mail method) in the VPOP3 settings and go to the Advanced Relay Settings tab. SMTP Return Address Modifiers section. In there, find the SMTP Return Address setting, and choose If the original address is not local, set to: and enter an address such as firstname.lastname@example.org in the box. This makes VPOP3 use the specified return address instead of the original one.
This won't change the way that replies are sent to the forwarded messages, but it will mean that if the forwarded messages bounce, the bounces will go to the specified address instead of the original sender.
Note that the return address you specify MUST NOT be forwarded to another email address, or you could end up with a message loop!
This article applies to any method of forwarding messages, such as the 'Forward To' setting for a user, the 'Assistant Setting', using distribution lists etc.
If this doesn't work, then any problems are probably due to other conditions set by your ISP/SMTP relay provider, so you should contact them for assistance. Alternatively you could switch to a different SMTP relay service which doesn't apply any 'mystery' conditions. There are several such services available (for a fee), and we can provide one for VPOP3 users if you wish. Contact us for information.
If you are forwarding mail to allow people to access their mail when they are not in the office, eg on their mobile phone, then it may be better to allow people to access their mail directly from the VPOP3 mail server instead.