why are emails returning as undeliverable ,sometimes

[Johnny angel]

my emails keep comming back as "undeliverable" and the only way I can get to
send them is to click the Send&recieve button first and then resend the
message

 

[Brian Tillman]

my emails keep comming back as "undeliverable" and the only way I can
get to send them is to click the Send&recieve button first and then
resend the message

It could help is you posted one of the NDRs.