You might receive an email that warns you not to reply without changing the reply address.
That's because we use Amazon Web Services to send our emails. That's lower cost and more reliable than other methods, but AWS requires us to specify in advance which 'from' addresses it should accept. If, for example, a ride leader sends you an email through the web site we normally have to change the 'from' address so it comes from emailer@portsmouthctc.org.uk instead of their personal address. We then tell you that you can't reply directly.
AWS accepts any address ending in @portsmouthctc.org.uk so there's no problem replying to messages coming from any of those addresses, and you don't see the warning.
When composing a reply, you can select the highlighted address in the warning message and paste it into the reply address. You can also right click the highlighted address, select 'Copy link' (or similar) and paste that into the reply address.
If you want to start a completely new conversation with the sender, you can normally click/tap the sender's name in the warning message.
By the way, we're aware there's a standard mechanism that allows us to specify replies should go to an address other than the 'from' address. However, some email software obeys that mechanism while others ignore it (presumably because spammers can use it to mislead you). Given we can't predict whether that mechanism will work for you, or not, we decided not to use it.