Using my BT email address I sent an email to a number of people. Every time one person does not receive the email from me. His address is a BT email. Others can do a reply all and it works fine for him, So address is correct.
Another BT user has the same issue with this person email all are BT addresses.
I use my BT email address for nearly everything. The only problem comes with the City of Edinburgh Council which has a fierce filtering system so contacting councillors and officers is always troublesome.
Presumably you are not a BTMail user. You should ask your contact who does not receive your emails to add your email address to his "safe senders list" which he can find by logging onto his email account and going to the settings found by clicking on his username at the rght hand side. Once there he should click on "Mail" then "Safe Senders" and after adding your email address click "save".
@gg30340 Thank you. Yes that's what I and many others have to do when contacting the City of Edinburgh Council. This is made more difficult as many Council officers do not publish telephone numbers and only some permit tracking via Outlook. Once you are on the safe list all is well until it is upgraded when we start again.