"Cannot send mail"

[#1]

I frequently get this message when abroad (or in Scotland) - never when at home. Recently the message also said 'the connection to the outgoing server "RELAY. plus net" failed. On another occasion the message " the username or password is incorrect" was given. This is infuriating and very inconvenient and inconsistent. Any help to get this round would be appreciated.  I have to add I am not very IT literate.

[#2]

Ok, let's start with the basics and go on from there.
1) What computer/device are you using?
2) What program/app is being used to send & receive emails?

[#3]

Thanks.

1) IPhone and IPad.

2) Outlook.

[#4]

so would it be correct to say that it works when you are connected via the Plusnet router but not when you are using the phone network or another ISP's internet

[#5]

It's intermittent - i.e. I can sometimes send emails when away quite normally (using Vodaphone or another ISP network) but then for no apparent reason I get the "Cannot send email" message. It never happens when we are using the PlusNet router at home. 

[#6]

Has this been resolved?

Generally this is caused by...

1. The SMTP configuration not being set to use AUTHENTICATION (same account details as per the incoming server)
2. The SMTP server being configured to use PORT 25 rather than 587

SSL/TLS must not be enabled.

If the SMTP is configured to use PORT 25, you will find that some networks will inhibit "off network" connections using PORT 25 - this is one technique used to inhibit email SPAM.

It would be useful for you to note how you are connected to the internet when this happens.  I have also seen these issues when connected via mobile data and travelling (as on the train).  One sees the problem, then it goes away - I suspect that this is down to faulty data services in some cells.

[#7]

Many thanks for the reply.

Both IPhone and IPad were configured to port 587 but I have changed the SMPT authentication  to 'Password' as it was already with the 'Incoming Server'. SSL was not enabled.

(I tried sending the failed emails both by WiFi and 4G without success.)

As the 'Failure to send email' message only happens when I'm away, unfortunately I won't know if this measure has helped until I go away again but nonetheless thank you for your help.