which.net publishes an SPF record that lists IP addresses that are trusted, and states other IPs are neutral.
An SPF record allows a receiving ISP to look at the incoming message and decide if it is trusted based on the domain name of the email and what that domain publishes.
Many ISPs in the USA (e.g. verizon.net) would silently ditch the email from you using which.net and sent from say Plusnet's relay server because the IP of Plusnet's server wasn't listed in the SPF record so you fell into the neutral category.
If it matters to you that your email is 90% likely to make it to the recipient, pick an email service that lets you receive and send through them from anywhere, and ensure your mail has a published SPF record. Even better get DKIM enabled.
The old university idea you could sent from any relay server (on the network you were attached to) is dead ; sadly thanks to spammers. But the filtering happens at the recipient, and even paying customers can't get big ISPs to change their email acceptance policy.
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