Trouble emailing and forwarding emails

I am using Runbox 7 on a Mac running OS 10.14.6 (Mojave). However, my problem occurs whether I use Runbox 6 or 7. If I try to email a friend who also uses Mac and Runbox 6 the email does not arrive. However, my friend can always email me . If I try to forward an email I get a message telling me the message has no address and can’t be delivered.

Hello @Tazzydevil and welcome to the community!

Does the problem only occur when sending email to one specific address?

Also, are you getting the error message when pressing Send or does the message send and then you get a bounce email message with the error?

– Geir

Hello Geir.

Thank you for your swift reply,

As far as I can see the problem occurs with one specific address only within Runbox. There has been similar difficulty with an account in Gmail. Also, the message sends and I get a bounce email message explaining the error. Often the explanation is that the original message does not have an address, even though I know I have included one.

With thanks.

Hello, again, Geir.

Further to my last reply, I tried to send myself an email, with the same result.