Updated FAQ

FAQ.md

@@ -1003,6 +1003,16 @@ If you are using a [VPN](https://en.wikipedia.org/wiki/Virtual_private_network),
 the VPN provider might block the connection because it is too aggressively trying to prevent spam.
 Note that [Google Fi](https://fi.google.com/) is using a VPN too.
 
+**Send errors**
+
+SMTP servers can reject messages for [a variety of reasons](https://en.wikipedia.org/wiki/List_of_SMTP_server_return_codes).
+Too large messages and triggering the spam filter of an email server are the most common reasons.
+
+* The attachment size limit for Gmail [is 25 MB](https://support.google.com/mail/answer/6584)
+* The attachment size limit for Outlook and Office 365 [is 20 MB](https://support.microsoft.com/en-us/help/2813269/attachment-size-exceeds-the-allowable-limit-error-when-you-add-a-large)
+* The attachment size limit for Yahoo [is 25 MB](https://help.yahoo.com/kb/SLN5673.html)
+* *554 5.7.1 Service unavailable; Client host xxx.xxx.xxx.xxx blocked*, please [see here](https://docs.gandi.net/en/gandimail/faq/error_types/554_5_7_1_service_unavailable.html)
+
 **Gmail errors**
 
 The authorization of Gmail accounts setup with the quick wizard needs to be periodically refreshed